If you haven't had a shower in a week, at least try to look like you have! This, to me, is a very very important tip. Ask yourselves, if you were to pick up a hitchhiker, would you pick up one who looks like he'll stink up your car? Most people would say no, they wouldn't. So even if you do smell to high heavens, at least look like you're as fresh as a daisy. Once they realize you've just rolled in the dumpster it'll be too late to drive off without you!
Also, have some gear with you, a backpack at the very least. This gives you purpose. Again, many who picked us up said they did so because we looked like travelers, not hobos. Chuck always stayed clean shaven and wore a travelling hat. He swears by that hat to this day. So go get one! It'll keep the sun off your face and make you appear all "worldly and stuff."