Was at its peak in the 70s, but is not very widespread nowadays. It is not recommended in numerous countries for safety reasons and should really only be done if your vehicle breaks down or if you are out of money. However, if you insist on using this method of transport, there are a few rules that you should follow.
Before anything else, enquire about the laws of the country you will be in. Next, choose the right spot: rest stops, toll booths, petrol stations and lorry parks for long distances. Opt for smaller roads for shorter distances. Wherever you choose, make sure you are in a spot where vehicles will have enough room to stop. Avoid carrying cumbersome luggage and don't forget to use a sign; it will let motorists know if you are going in the same direction.
Finally, try to refrain from hitchhiking at night, especially in regions with which you are not familiar, and even more so if you are a woman. Moreover, work as a pair when hitchhiking. Apparently, male-female couples are the most reassuring...
Don't leave your bags in the car if you are not in it, and if possible, keep them next to you rather than in the boot.
If you feel that something about the driver is off, avoid all doubts and get out of the car when the occasion arises. The same goes for his driving. Try to pay attention to where you are to be sure that that driver is going where he said he was.
Be cool, friendly, and don't forget to make conversation; this is usually why motorists stop to pick up hitchhikers.