Petrol station hitchhiking
Petrol station hitchhiking, as well as hitchhiking on service stations in general, is a great way to travel for free in countries where hitchhiking on highways is forbidden. It generally works best if the number of petrol stations on a highway is around 1-2 per 100-150 km (?). On most petrol stations you can easily find cars going your direction, although you have to be aware of highway splits, too.
Sometimes it can be useful to get in touch with people who work on a petrol station. Asking drivers directly instead of waiting for the car at the area's exit is a good way to catch a ride. It is recommended that female hitchhikers first ask the male driver(s) where he (they) is (are) going to, instead of revealing one's own destination in a question to the driver, so that during the conversation it would become clear if it is safe or not to go with that driver, and if it feels risky, one can pretend not going the same direction.
Starting a conversation will give drivers more chances to look at you as a person. It is less likely then that their answer will be negative if the direction of yours and the driver's destinations matches.
While hitchhiking between country borders through petrol stations, it is a good idea to check the oil prices in the source and the destination country. If the destination country has significantly cheaper oil, many cars going in your direction might choose to avoid the petrol stations near the border.
Germany is especially good for this. In France it works slightly worse (even though still good enough) because of the higher number of petrol stations per given distance, thus relatively less cars per petrol station.
When in Germany, it is generally recommended to avoid the so called Autohofs which usually are not great for hitching a ride mostly because they are not located directly on the Autobahn and it is hard to get a ride from there in exact direction of where you are heading to.
Autobahnraststätte is totally different: There you find both a petrol station and a restaurant for all sorts of people who need a break - an excellent place! You should pack a map with all service areas and study it. Alternatively you can download a pdf here with all service areas and onramps on it and combine it with your smartphone.
In Italy you have to be careful: it is illegal to hitchhike on the Autostrada, and it is a bit unclear if it is illegal to do so on the Area Servizios (petrol stations with restaurant on the Autostrada), but in all the years he's been using them (1991-2012), Prino has never ever encountered problems on them.
In The Netherlands Shell has the policy to ask hitch-hikers on their service stations on the motorway to stand behind the services or next to the building. The same is applied by some of the BP petrol stations in the west of the Netherlands. It is just better not to try to talk with everybody!