I'm not an expert but I will share my small skills on the art of hitch hike. Pretty please: If you got any more tips comment this post, so one day we will have a full list :)
1) Before leaving do some "homework": ONE: check in google maps your route and write down a list of cities you will pass by, plus how long does it take from one city to the other and how many kilometres. TWO check out the suggestions in HitchWiki;
2) When you are hitch hiking be flexible and ask people (in gas station for eg.) about the list of your route: locals know better then us. (The faster doesn't mean the better);
3) Collect cartoon boxes from supermarkets or any shop and invest in a good quality market, I like either black or red colours, because they are catchy. Make written signs as neat as possible of the place you need to go;
4) Try not to ask a ride futher then 2 hours, sometimes you have better chances if you go "with little steps";
5) For your security, when a car pulls over to give you a ride, first go talk with the driver and make 5 seconds "evaluation", if you feel a good vibe, go grab your backpack. If not, find an excuse. You need to read people's eyes to commited to enter their cars, you need to feel safe.
6) Helps a lot if you know the language of the country, if not, don't worry! Body language works. Also you can show your list, you never know if the driver can drop you off futher then your written sign.
7) If you are making long distance journeys besides of taking snacks and water with you, make sure you are ready to: sleep in the bus station or even on the street or in case your credit card doesn't work you will starve a bit. But smile!! You can be lucky enough for driver asking if you wanna stay in their place or offering you a meal. Your survival spirit will be your best friend!!
8) Don't panic if you can't get a ride for 2 hours, there's always someone who will take you even if it takes 8 hours! If it's taking a while look around you, maybe you are in a bad spot or you need to ask for a ride closer.
9) The choice of your spot is very important: has to be a spot with enough place for a car or a truck to stop. Don't hitch hike in highways because it's forbidden to stop and dangerous, you better do it in gas stations;
10) If you are hitch hiking in the gas station and no one stops, try to talk directly with people. Sometimes they are afraid of taking strangers but face to face conversation can show them you are truthful.
11) BE PATIENT!!!
12) Try to avoid night Hitch Hiking for your safety;
13) Remember that bad thing can happen if you are in the wrong spot at the wrong time. Actually this can happen anywhere in the World.
14) Listen to music while you are waiting, time will fly!
Hitch Hiking is such an adenturous way of travelling: you meet tons of people, you create connections, you will have tons of fun stories to tell.