When a car stops ask the driver where he is going to. At this point it is easy to decline the lift, if you don't like the look of the driver or if he isn't going your way. Never smoke in someone else's car. The lighter your load, the quicker you travel. Take an international drivers licence.
Many people stop because they want someone to share the driving with.
Although hitch-hiking is more hazardous than train travel, you have a greater chance of being attacked walking the streets of a big city in North America than hitching in Europe. The most dangerous thing about hitch-hiking is the possibility of being involved in a car accident or being hit by a car if you stand too close to the side of the road. You run the risk of a car accident everytime you get in a car, although this is multiplied when you are hitching because you don't know what the driver is like. Despite the perceived danger, there are plenty of ways to minimise your risk. If you're a single female you'll travel quickly, however you'll also attract a lot of weirdos.