Hitchhiking at night
Hitchhiking at night is definitely not as easy as during daylight. But it's still possible, depending on the situation and location.
Sometimes, especially but not exclusively at night, drivers will pick up a hitchhiker because they are tired and sleepy, but must get to their destination on time. Make sure you never fall asleep in someone's car if you are suspecting this might be the case. Even if you can't find a conversation topic, stay alert. If you have a driving license you might indicate your driver of this fact. In some cases he or she will be happy to let you drive.
To enhance your visibility, and therefore also your chance for a ride, wear a fluorescent vest or jacket. You can buy these in any bigger gasstation, especially in those located on highways. They can be pretty expensive in these places though (they can ask up to 4EUR for one), but if you buy one in a regular supermarket, you'll find one for 1EUR. If you don't have a vest or jacket, try to lay hands on any kind of fluorescent of reflective material; reflective strips that bikers use for instance. Apart that it helps for visibility; it also looks more "professional" and trustworthy. Drivers tend to trust you more, because if you'd be a maniak killer on the loose or an escaped convict or something, you wouldn't wear anything that draws attention, right? In some countries in Europe, like France, there are laws that force you to wear a fluorescent vest. However in normal circumstances the police won't bother to stop.
Some hitchhikers also use flash or blinking lights to draw the attention of drivers. This can work rather effectively. When done in the right way, that means to start using the flash when an approaching car is about five seconds away, most drivers drop their speed for example from 80 to 40 kmh. They are very attentive at that moment and especially keen on getting to know if they will be obliged to stop (by police for example). When done at the very right moment, a decided gesture will bring some of those drivers to stop. The technique has been described in detail on this page of VHHC.
In any case, try to stand in a well-lit place. Traffic lights are also OK.