I have successfully hitchhiked in western Europe, mostly France. It can take time to find a ride, but I (young middle class white male, clean shaven) was always picked up by ordinary folks just happy to help out. The trickiest part is finding a spot where people have the time to see you and slow down- in my experience, just before highway tolls is best.
I had basically the same experience. I've done a trip from Riga across north-east Europe down to Italy. Two things stand out in my (short) experience:
1) It was incredible easy to talk with people that did not speak your language.
2) I meet far more women than men. Even alone women and predominantly from North Europe.
Gas station is not great because people generally don't like being approached; they think you're going to ask for money, ostensibly for gasoline. It's also more difficult to target people going the direction you're going.
Truck parking might work if you're looking to get to the opposite coast, but a trucker isn't exactly going to be super amicable about dropping you off in the middle of the metro center of the next city over. He's got a schedule to keep.