These are just some basic tips and simple words of advice for hitchhikers. A lot of it is just common sense but some of it I had to learn the hard way. They are basic but, I believe, pretty universally applicable regardless of where you are in the world.
In no particular order:
Sign's do help! Yeah you'll feel like a bit of a clich but if you're on the open road and you have the means to make a sign to let people know where you're going it really will make more people inclined to stop. If there's lot of places on the way you may even want to write a nearer location on the sign to increase the number of people going as far as you are and you can always strike up conversation and see if they're going further.
Don't settle for the first offer! This is hard especially if you've been waiting for a long time with no luck and, ultimately, it is really a judgment call based on where you are and the amount of traffic but you have to firmly keen in mind that it won't benefit you to be stuck in the middle of nowhere even if it is several hundred miles closer to where you want to be.
Introduce yourself! Seriously you have to do this as soon as you're sitting down if not sooner. It's more than possible to miss the moment and then you're both complete strangers.
Talk! Lot's of people pick up hitchhikers because they want the company in order to stave off fatigue or boredom. Of course you should gauge the situation but in most cases you'll be expected to provide at least light conversation.
No sleep till Brooklyn! You can't fall asleep when you're in someone's car or truck for a variety of reasons most of which are so obvious they really don't need to be stated. Suffice to say it's not the done thing. As mentioned above some people pick up hitchhikers to prevent falling asleep at the wheel. The last thing they want is to hear your snores.
The sights and sounds! You're going to hear and see a lot of strange things. There's few other ways to find out just how varied people can be and you'll get all sorts. The golden rule is you want to keep conversation light. No politics or other polarizing topics and if the driver brings such subjects up you'll want to be cautious and neutral. Most of the time you'll be getting rides with truck drivers and it's important to remember that many of them almost live in their trucks so you're essentially in someone's living quarters. There are no certainties with truckers but it's not uncommon to see pornography and drug use. If that bothers you... well prepared to be bothered.
Cell phones! It helps to have one and doesn't hurt to subtlety show it off. This is just a basic safety thing. If the driver knows you have the means to contact people you're a little safer with the odd shady character. Its bad etiquette to be on the phone in someone else's car but one way to stay safer is to arrange for someone to ring you and let them know what you're up to. A driver is less likely to mess with you or waylay you if they know others expect you to arrive at a certain time.
Hitching with others. Yeah no exclamation marks on this one. Hitchhiking with a friend makes a world of difference from doing it alone but it does alter things. Before we go on it should be more than obvious that three really is a crowd and more than three hitchers is ridiculous.
Now obviously lone drivers are less likely to pick up two people but I think gender plays a part. Hitchhiking is a situation where, by necessity, people need to make snap judgments so factors such as gender, race, age and general appearance can and will play a part. It's sad but true. Anecdotal evidence shows that two males are going to have a hard time getting a ride together and would probably be better off splitting up. A male and female will have inordinately more luck and the same again applies for two females.
Appearance! Briefly covered above and I think it's pretty obvious but the more presentable you look the better. It's not a job interview sure but some effort helps.
The bare necessities. You'll always be out on the road and in the open more than you'd expect of hope for. With that in mind bring proper clothing, sunscreen, hat, sunglasses (keep in mind the appearance factor though), umbrella etc. Also take a map!
Who's afraid of the dark?!? If you're hitching you should be. The dark is really bad for hitchhiking. People just want to get where they're going and they get less chance to check you out before stopping. The dark is also cold and sometimes wet and... well dark. It sounds trivial but believe me if it's starting to get dark you need to start looking for a place to stay. This one really ties in with making sure you get rides that will drop you off where you want to be or in a place that is busy enough to give you enough traffic for more lifts.
Location, location, location! First of all you need to think how most people are going to drive to where you want to go. By this I mean what is the most common route. The best place to start hitching is as far along this route as you can manage to get by foot, bus, train etc to rid yourself of as much suburban, city and short-trip traffic as possible.
Once there you want a place with a lot of traffic where people can easily pull over. This isn't going to happen in really busy fast spots or places with not space to pull over. Maps can help you pre-plan this or ask around and see the next point.
Truck stops! They are the be all and end all. Truck drivers are ideal because they often like a bit of company and they travel in very long stretches. As with all demographics some are unsavory but most are nice people. Raising your thumb a little down the road from a town's largest truck stop should get you a ride in no time. If you're bold you can even go in and ask people for a ride.
Legality! It seems like its different everywhere. It doesn't hurt to know what it is in your area and also if it's actually policed. The shiny flip side of this rather tarnished coin is that police (in some locations) are sometimes obliged to give you a ride home if you have no physical way of getting there. Apparently the only downside is that sometimes they'll make you fill out paperwork or take their time.
That's about all I have at the moment. I'll probably add more things later as they occur to me. As stated at the top these are just basic universal tips to help you out.