Rubbertrampings obviously the most free, yet you have the downside of paying for gas, which'll probably hit 5 bucks a gallon this summer. (I mostly drove my ass across the country when travelin) Hitchhiking is iffy, all depends on where you're at. A much more reliable method I've found is using craigslists' rideshare feature, I've meet SO many cool peeps that way, plus you dont gotta thumb it all night in the cold. As for hoppin trains, don't. Its no longer what it used to be, there's alot less freight being moved and more hoppers on each train because of that; almost every yard nowadays is a humpyard. One of the coolest cats though I met was a teenager whod already ridden i think 6 trains, so I suppose it all comes down to luck and willpower.
Obtaining a notarized letter from your parents, expressing thier knowledge and approval of your travels and explicitly permitting you to crash with people along the way. (Many crash pads are closed to those under eighteen for legal reasons...this kind of letter will open them up.) A parent or guardian must sign the letter in front of the notary. The letter then becomes a legal document.