Hitching has been around probably since man built the first cart. The aim to get from A to B using someone else's transport costing you nothing. Undoubtedly this is a very environmentally way of travel you fill a seat that would normally be empty. It can also be a good way of meeting locals and gives you a means to reach remote areas where public transport does not exist.
Hitching is not a safe way of travel. In general if you are on your own, especially if you are female then you should not hitchhike. You are also putting your life in the hands of the skill of a driver who you don't now. Never except a lift from someone you thing may be drunk or under the influence of drugs.
A passing motorist has seconds to decide wether to give you a lift so its important to make yourself look respectable. Dress smartly but causally, choose a road leading out of town in the direction you want to go. Use a sign with your destination. If a driver can only take you part of the way check where he is going to drop you off, is it in a town will you be able to get a ride out, if its at a junction is it in the middle of nowhere.
In general the sign for a lift is holding you hand out with the fist closed and thumb extended. However in some countries this is an obscene gesture so check and if you are unsure just raise your arm.
Good countries for hitching are Germany, New Zealand and most of Eastern Europe. Poland is particularly good as the government provide insentives for picking up hitchhikers!
Bad Countries are Australia, France, Southern Europe, Scandinavia.
United States of America getting a lift is often not a problem but the chances are they'll be a religious fanatic or weirdo.
Hitching in the developing world is very much a way of life, the main difference is that you will be expected to pay to cover your share of the fuel. The other difference is that you may be cramped in the back of a truck with pigs, chickens and as many other 'hitchhikers' that can be squeezed in. This is undoubtedly an experience but you need to be prepared as you may be out in the open under the midday sun so hat and plenty of water is essential.
Even if you are not asked you should offer to pay for your trip. In general you shouldn't pay more than the equivalent bus fare, as with taxis agree the fare before you leave but don't pay until you reach the agreed destination. There have been cases of people being taken part way and then when in the middle of nowhere being asked for more money but this is the exception rather than the rule.