How to hitchhike

Regardless of what your friends may say (and don't even think of telling your parents), hitchhiking is a viable means of transportation for the adventurous impoverished. Be warned, though, there is nothing to make you feel more like a repulsive steaming turd than the continuous stream of rejecting cars you are sure to encounter. Hitchhiking will make you wonder what is wrong with you, why 99.9% of drivers distrust you at first sight. Then a stranger's car will pull up beside you, and all will be right with the world. Here's how to do it:

Materials

If you will be making a sign, you'll want to scavenge a piece of cardboard large enough for approaching cars to read, plus a heavy marker (or pen, or lipstick, whatever). I've found that a sign does not increase the speed with which I'm offered a ride, that it gets annoying to carry around, and that it makes lying to cops- "no, officer, of course I'm not hitchhiking"- nigh on impossible.
If possible, some bribe material- gas money, chocolate, etc., though not drugs, as there is a pretty good chance that you'll be stopped and searched by cops. I don't recommend bribing your driver with sex, but I don't recommend against it either.
If its cold: Element appropriate clothing. Particularly gloves. You will be standing out in the elements after all, and waving your thumb in the wind.

Preparation

First of all, decide where you're going. This can be across the country, or to the grocery store. If you choose to make a sign, one large word works best:
"ANYTOWN"
Secondly, decide the route you want to take to get there. The idea is not to recreate an authentic Jack Kerouac experience (itself a somewhat tenuously "authentic" construct), and so I would highly recommend availing yourself of mapquest or mapblast or a good highway map, instead of casting yourself like a dandelion seed into the wind. You need to know which highways you are taking. If you don't live immediately near a highway, know how to walk to one- the right one. Use common sense- you won't get very far north standing with your thumb out on a south-bound highway. And your journey will probably require changing from one highway to another at least once, so you most importantly need to know where you can tell your driver to let you off.
Find out if hitchhiking is legal in your area, if you care about that sort of thing. In most U.S. states it isn't, but if you've got no criminal record and you aren't wanted for anything, you can always play dumb, at least for the first time you're pulled over. What is important is knowing, in states where hitchhiking is legal, what the stipulations are on that legality. Usually you won't be allowed to hitchhike on interstates, or turnpikes, etc. This is not a problem because waiting on entrance ramps, instead of on highways themselves, is better strategy anyway. On the latter most cars will be moving too fast to stop for you, even if they wanted to.
Don't look scary, if you can help it. For example, if you're like me, take that big safety pin out of your nose. Us adventurous impoverished usually have to be somewhat accomodating to our kindly drivers' anti-punk prejudices. The need to that same sort of patience applies for as long as you're riding. Be friendly and personable, and not too honest. Telling you're trucker host he's a pig due to his right wing politics, after he's gone out of his way to pick you up, is just bad manners.
If you are female, you walk a fine line between attracting drivers' interest and getting raped. Drivers, even those who have no plans to sketch on you, will be less threatened by you and more willing to pick you up if they can tell as they drive by that you're female. You may choose to advertise your gender by wearing a bright colored skirt, but going all out and wearing something skimpy is ill advised.
Carry a concealed weapon. I like butterfly- or hunting-knives, some might prefer guns or even pepper spray. Whatever you carry make sure that it is simple to retrieve and you know how to use it.
If you feel like being safe, tell someone where you're going, and bring a cell phone in case for some reason you don't get there.

The act itself

Go alone. If you've got a little child, that's ok, even beneficial. If you're got a dog, it'll probably mean fewer people will be willing to pick you up, but it's still ok, especially if you can expect the dog to protect you. But if you've got another adult with you, then almost no single driver will pick you up, for fear of being overpowered, robbed, and killed. And cars already containing passengers tend not to pick up hitchhikers as often as single drivers do, due to the slim odds that everyone in the car will be individually inclined to do so.
Be prepared to wait. If you've got your nose in a book, or sit down, or look otherwise comfortable, then you're less likely to be picked up. But do bring your discman or your patience.
Be prepared to jog. Unlike the movies, a car zipping along the highway just can't pull up alongside you. It'll have to decelerate and will end up considerably ahead of you, at which point you'll have to cross the intervening distance without making the driver wait for too long.
If you've got to be someplace at a specific time, leave yourself extra time to get there. If you're on a moderately busy road, you probably won't wait for a ride for more than 20 minutes on average, but you never know when you might get stuck for hours.
Try not to wait on a tremendously busy road. The Kitty Genovese theorem applies here: if there are many people who could pick you up, then nobody will. Of course, a deserted highway means no cars. No cars means no ride. Entrance ramps tend to be good, especially if they are preceded by traffic lights, forcing cars to stop. Many ramps are curved though- don't stand just behind a bend or you may get mown down. And in cases off exceptionally low traffic at your chosen entrance ramp, you may still be forced to try the hghway.

You can expect to be picked up by

50 year old nostalgiac businessmen in four-door sedans, who will talk to you about their glory days, and try meekly to get laid.
Hippies in crumbling Volvos.
Christians in tremendous pickup trucks, who will attempt to draw you into Bible discussions. As nixnutz points out below, pickup trucks and, I would add, vans, especially old ones, do tend to provide a very disproportionate number of rides.
Truckers hauling pointless commodities to mid-sized cities, who will talk your ear off because they're bored.
Young couples with dogs.
Older couples who are dearly concerned for your safety and will insist that you call them when you reach you destination. This has happened to me, though only once. Admittedly, older couples usually fall into the following category.

...But probably not

Parents with small children in minivans.
Frat boys in convertibles.
Teenage girls in SUVs.

Now hold up your sign and smile your most charming smile. If that tactic doesn't seem to get you very far, look pathetic and cry.

The Pros and Cons of Hitchhiking	Kitty Genovese	Starbucks = free milk	Travel changes your brain
hitchhiking	Subaru SVX	old green field jacket	Jack Kerouac
Kerouac's On The Road Manuscript	On the Road	Round Ireland with a Fridge	Survival isn't good enough, you have to live
gun rack	The Dharma Bums	Neal Cassady	Mexico City Blues
Some of the Dharma	gutterpunk	Crazy Horse	The First Third
November 19, 1974	Chevy Van	teen drinking

"ANYTOWN"