Pack light.
Be (or at least look) as clean as possible.
Expect to walk a lot. Plan for it unless you don't mind staying in one spot for a while waiting for a ride.
Don't do ANYTHING without the driver's permission. Ask before rolling window, smoking, taking your shoes off, etc.
If a driver wants to talk, talk. That's a very fair payment for a free ride. If they want silence, don't bother them.
Don't be afraid to turn down a ride if you have bad vibes.
Most truckers drive for companies. Companies don't allow passengers, so very few will pick you up.
Lots of water! Lots of calories! If you're walking a lot with a pack, you'll need these!
Walking the interstates makes it easier to get a ride, but it's illegal. Weigh the risk. You WILL have cops pull you over. If you have problems with authorities, don't bother. You WILL see them. They'll usually drive you to an exit or a truck stop/gas station if you're able to keep a cool head.
I'm bored of writing this. If you have specific questions, ask and either me or someone else will happily answer.
Good luck!