I gathered some hitchhiking experience recently and I'll share things I found out here - hope it helps you! You are free to link to this page as you like, but I'd appreciate if you linked to the top of my home page instead, though (rest assured that I won't sue you or something for linking this guide directly, though). If you do link (or merely like) it I'd be happy if you emailed me, for I'm curious about my audience ;-)

I'll give some general advice on best practise I found to work well for me and list some good hitchhiking spots I encountered on my way. Stay tuned for more.

Tips

Useful stuff to carry

(I mentioned most of this stuff below, but I'll list it all in one place here)

A good map with all the motorways/major roads
A notebook and some pens (one will eventually run out of ink)
Black magic markers (at least two)
Several pieces of cardboard for destination signs
A tent, sleeping bag and foam mattress
Water/food
A flashlight
Band-aids (best as part of a complete first-aid kit)/paper handkerchiefs
String, duct tape, sewing kit (best with a strong, sharp leather needle so you can easily sew several layers of fabric together) for all sorts of patchup jobs
A hat to protect your eyes from the sun (squinting because of the sun isn't exactly helpful in smiling at potential rides - neither are sunglasses because drivers can't see your eyes)
A multi tool/swiss army knife
A cellphone/cards or coins for public phones
Rain gear (including waterproof pants and boots; I suffered the weight penalty of carrying full leather gear and was pretty glad about it occasionally...)

Have a notebook

Carrying a notebook and some pens proved invaluable on countless occasions. Originally I brought it along to write my travel log. Apart from that I used it to note down contact addresses and numbers (paper doesn't run out of power as your mobile phone will eventually...), reminders of things to do, useful expressions in several languages, hitchhiking spots, the lyrics and guitar tabs for 'Molly Mallone' and numerous other things. All the stuff I accumulated in there saved my ass several times!

Get a good map

This shouldn't need to be mentioned, but as I met some hitchhikers without one I'll mention it anyway. At the very least it should have all the motorways and major roads. Rest areas/gas stations are a bonus, but at that resolution you won't cover a big area (I got a map of Europe that lists only motorways and major roads but no rest areas). Apart from plotting your course it's also invaluable in communicating with drivers who don't speak any of your languages. If you're in Germany you can usually get a free directory of all the rest stops at any of them.

Be ready to hike

The 'hiking' in 'hitchhiking' is there for a reason. Occasionally you'll have to cover quite a distance on foot (ending up in a bad spot, getting lost on your way to a spot, getting dropped off in the outskirts of a town), so be prepared:

travel light: Depending on your strength/stamina (and on how much of a masochist you are) you shouldn't carry more than 30 kilos. Even more important than lightweight baggage is proper distribution: If you can fit everything in your (single!) backpack, by all means do! I carried a carry bag (nothing else being available) as well as a second backpack (having learned from the carry bag experience) slung over one shoulder, and I cursed both for every single step I had to carry them (the carry bag put lots of strain on the forearms/triceps/shoulders, the slung backpack only strained the shoulders, but chafed like hell after some time). Also getting in (on busy roads for example) and out (I hope you never get into that kind of situation, though) of a car quickly is greatly faciliated by light and compact baggage.
mind your feet: Wear proper, well fitting shoes, (hiking boots, running shoes, combat boots if you really must - flip-flops are definitely out of the question), preferably broken in already.
Apart from proper shoes you should definitely have some band-aids and paper handkerchiefs for patching up sores and blisters (paper handkerchiefs are sometimes absolutely sufficient, provide more padding and don't leave sticky residue like band-aids). Even if you have good, broken in shoes better take that stuff along, chances are your feet will still get sore after extended periods of wandering!
water: Most hitchhiking takes place in summer, so - depending on the region you're in, of course - you can usually expect a hot and dry climate. If you do any serious walking under those conditions you can expect to get thirsty real quick. Carry at least two liters of water (definitely more if there's any chance you might have to hike for extended periods of time) and refill at every opportunity (at least in Europe you can ask everywhere for tap water, and usually it's drinkable (it's heavily chlorinated in some places and/or might taste a little foul, but that still beats dehydration) for you never know when you can get another refill.
food: Definitely try to get some food (cookies, bread, dried meat pretty much anything that will keep) before you head out. Having something in your stomach definitely helps a lot in keeping you going, while an empty stomach will make you feel weak and turn every step into an effort after a while.

Don't hitchhike before motorway junctions

Try to hitch a ride that gets you onto the motorway in your direction. That way you'll get all the traffic going your direction instead of just the fraction that goes through the motorway leading up to the junction.

Use a sign

There's some purists out there who don't believe in signs, but your chances on a ride will greatly increase if potential rides know in advance if they can even help you by stopping for you. Write big, bold letters, so it can easily be read from a distance (like a street sign).

Have a backup plan

Be prepared for problems that might crop up:

rain: you might have rain gear, but do you have a transparent plastic sheath for your sign? If not get one, otherwise it will get soggy and unreadable in no time; also markers will hardly write on even slightly wet cardboard, so try to keep it dry and find some cover while writing. Duct tape will help in a pinch to waterproof cardboard, but prepare it in a dry place as markers will write as well on wet duct tape as they do on wet cardboard: not at all.
destination sign materials: Always carry two markers, eventually one will run out, and you don't want that to happen to you in a spot where you can't get a fresh one; also have several pieces of cardboard, in case there's more hops than you originally planned)
tent: If you have room to spare in your pack definitely get a tent. If you get stuck somewhere along the way and need a place to sleep you'll be glad you brought it. Maybe you can get away with sleeping out in the open, but it might rain. If you want to sleep out in the open it's probably a good idea to get a military issue sleeping bag that's rated for cold temperatures.
contacts: Use Couchsurfing or Hospitality Club (for this one you should register way before you start out, new applications are processed by volunteers and can take a long time to get approved) to find people along the route who might be able to host you if you get stuck. I usually try to find some phone numbers that way, as you tend to arrive in town on pretty short notice when hitchhiking, and email usually takes to long to be noticed in time (unless you're willing to spend several hours and lots of money in an internet cafe). If you can afford a hostel definitely try to look up directions/phone number in advance.

Smile!

Try to look friendly and cheerful; smile at drivers passing by, even if you've been waiting for a while already. More often than not potential lifts expect to have an interesting conversation with you and scowling at them won't exactly give them the impression of somebody in the mood to talk.

Travel in small hops

One of the biggest mistakes you can make is writing a faraway destination on your sign. That way you'll get lots of sympathetic shrugs but hardly anyone able to take you along all the way. If you know there's another good spot some (not too great) distance away, definitely try to hitch a ride there and use it as your next stepping stone. Depending on where you are 50 to 100Km should be ok. If there's few junctions/major cities along the road to your eventual destination you can probably double that distance.

Trucks

Hitching rides with trucks is mainly useful if you're travelling alone (usually there's only one seat available). If you meet that criterion it can be a great way to cover great distances. There's some things that will make it a lot easier to hitch rides with trucks (some of these might be specific to Europe, I don't have any experience with other places, yet):

timing: Truckers have to pause for an hour for every four hours of driving. After ten hours they have to sleep for eight hours before they can set out again. Therefore 0500 to 0600 in the morning is usually a pretty good time to hitch rides with truckers as they'll set out as early as possible after their mandatory rest period. Accordingly later than 1800 is an extraordinarily bad time to hitch rides with trucks, as most drivers who pull into a rest stop at that time will stay there for the night.
strike up conversations: Truckers who aren't going your direction might still be able to help you out with their radio (asking fellow truckers still out on the motorway if they'd pull into the rest stop to pick you up), tips (which roads to take, what to write on your destination sign), maps or simply encouragement and sympathy (very helpful if you've been waiting for some time already...)
Weekends: Lots of trucks are going back home on weekends. So if you can figure out the location of their home base somehow (license plates, contact information on the side of the truck) chances are they'll be going there.

Plot a course before you hit the road

Maybe not much in the hitchhiking spirit, but I got pretty burned in Budapest by hitching a ride to an on ramp that was closed due to construction. Try to find out about these kinds of things before you move out and try to find an alternative route. Also trying to find out about good hitchhiking spots along the way (ask locals, google for them, use Hitchbase) and noting down their location helps a lot. If you don't scout for a good spot in advance you can easily spend several hours looking, all the while carrying your (probably heavy) pack. And each of those hours will get you closer to that magical nightfall boundary...

Other hitchhikers

Occasionally you might meet other hitchhikers along the way. They might be able help you out with tips, maps or encouragement, but nonetheless you are usually competitors, so you might want to keep a few things in mind in the interest of good relations:

If you arrived later stand in line/pick a spot further downstream. You might still get a lift earlier if you look nicer :-)
Whatever you do, don't stay too close. In the eyes of passing cars this will seem like a (larger) group of hitchhikers and this will decrease both your chances of getting a lift
If you are in a rest area maybe you can try to come to an agreement on who gets to look for lifts in which parts of the rest area

Spots

General advice

Picking a good spot is the essence of hitchhiking. The more of these characteristics a spot has, the better:

Lots of traffic
Slow traffic (highway rest areas are particulary good in that regard, usually cars will not go much faster than walking speed there)
Ease of stopping (hard shoulder, stopping bay, wide tributary with negligible traffic, wide inlet, parking spots)
Good visibility (no trees/boulders/whatever keeping drivers from seeing you), a long, straight stretch of road so drivers see you from a long distance
Located in a populated, built-up area (it's harder to get a lift out in the sticks, drivers will wonder exactly why you ended up in a deserted place like that)

List of spots

I'll only list fairly good spots here. I used several others not listed here, but in retrospect I think that I only got lucky there, so I won't bother with those.

Bayreuth, Germany

North (Leipzig, Berlin): Try to get to the truck stop Himmelkron, if possible (it's about 15Km North of Bayreuth, on the A9). It's probably best to use the on ramp Bayreuth Sued (42). The on ramp itself is pretty busy and there's no good spot to stop, but there should be a suitable spot somewhere between the University and the on ramp. Sorry for the vagueness, but I didn't try to hitchhike from there, yet.
South (Nuernberg, Regensburg, Munich): There's lots of local traffic going to Nuernberg, so your best bet is probably hitching a ride from Bayreuth Sued to Nuernberg and move on from there.
Southeast (mainly Weiden): Take the Bus (or walk) to Aichig and walk a little past the crossing Kemnather Strasse and Frankenwaldstrasse. There's a broad side street leading up to a bunch of houses. It's a good spot to stop (dead end, so hardly any traffic coming out; good visibility), but you might be in for quite a wait as there's not much traffic passing that way outside the rush hour.

Bratislava, Slovakia

Budapest: Try to get to the Hungarian border (A friend from Bratislava took me there, so unfortunately I can't help you with directions) and stand directly behind the border. It's probably better to try and get to the Hungarian side instead of standing in the no man's land right behind the Slovakian border post like I did.

Brno, Czech Republic

Bratislava: Get out to the Ikea (there's a free bus line going that way) and walk to the Roundabout. You can either stand at the roundabout or directly ask for a lift at the adjacent truck stop.

Budapest, Hungary

Balaton, Vienna: Go to the Kelenföld train station. Use the underpass to cross to the eastern side of the tracks. Head straight until you hit the big main road. Walk along that road in the inner city direction and use the overpass to cross it. Walk back on the other side until you get to the gas station.

Hamburg, Germany

All directions: Try to get to the rest stop Stillhorn. Best way to do it is probably to go to Wilhelmsburg by S-Bahn (line S3) and take the bus from there (don't know the bus line, but the bus stop is called "Kirchdorf Sued"). There's a rest stop on both sides of the Autobahn and an underpass to cross safely. Use the one on the west side for southern destinations, and the one on the east side for northern destinations.

Koeln, Germany

South (Frankfurt): Take the U-Bahn U9 to the last stop (Koenigsforst), exit the train on the starboard side, and turn right into the street leading out of town (Heumarer Mauspfad). Walk along the road till you get to a T-junction, where you turn left. This street leads to the on ramp and there's a stopping bay along it where you can stand. While you might not get a ride to Frankfurt, getting one to the next big rest stop (Siegburg) shouldn't be too difficult.

Munich, Germany

North (Berlin, Nuernberg): Take the U-Bahn line U6 to the station Studentenstadt. Exit the station through the east exit (Ungererstraße) and turn left. Walk straight across the big intersection, after 100m you will get to the A9 on ramp.

Prague, Czech republic

Brno, Bratislava: Take the Metro line C to Chodov station. Exit the station through the main entrance, turn left and down the embankment. There's a rather quiet tributary (so it's easy to stop there) merging into the motorway. At the time I was there, there was construction going on and the motorway was reduced to one lane. Thus traffic was going pretty slow. That's probably temporary, though.

Rotterdam, Netherlands

Utrecht: Try to get to the A20 exit 14. It's a walk of about 2 hours or a tram ride on line 5 to either 'Abraham Kuyperlaan' or 'Schieweg' from the center. There's a hard shoulder/stopping bay on the on ramp.

Tilburg, Netherlands

Eindhoven: Use the A58 exit 10 (Hilvarenbeek). There's enough space to stop directly on the on ramp. Don't even think about using exit 11 (Goirle). There's nowhere to stop and it's incredibly busy. I cannot really provide you with directions as I'd picked exit 11 at first and walked roughly along the motorway to exit 10 on discovering that exit 11 was not viable. You don't really want to do that, it's at least 2 hours of walking. There's a bus stop pretty close to exit 10, so you can probably easily get there by bus.

Utrecht, Netherlands

Den Haag: There's an official hitchhiking spot (Liftensplaats) for destinations on the A12 down in the Southeast. I walked there (about two hours from the center), but there's a bus from the Central Station as well, the bus stop is called 'Stadion Galgewaard'.

Vienna, Austria

Linz, Salzburg: Take the Metro line U4 to the last stop (Hütteldorf) exit the station on the port side, turn left at the exit (Keißlergasse) walk for 200 meters and use the underpass to cross under the tracks. Walk left, cross the bridge and turn left. Walk for 2km until you get to the overpass across the motorway. Cross it, and set up at the gas station 100m to the right.