I got a real good one right here:
As you're stepping into the car/truck, saying ur "thank you's" "where u heading?" etc, pull out ur cell phone and start pushing buttons, really conspicuously. Then if the driver starts creepin/ threatening u, tell him that as you were walking up to the car you sent his license plate number to your mother/someone who cares about your safety, and if they dont hear from you within x hours, they're sending the plate numbers to the police etc. Of course its all a big bluff, but better yet if you're really worried you could actually make an arrangement with ur friend/mother and do the license plate thing for real.
I never really do that I'm a guy and i've never really been worried hitching in the US but i advise all my lady friends to do it. Besides my main knife I carry on my belt, I have a crappy little Wal-Mart lock knife that cost me 74 cents which I keep in my boot, it fits perfectly and its discreet. And always a solid metal pen or sharp pencil in my jacket pocket which would do the job on someone's neck if my life depended on it.
One time I was hitching through the countryside in Oregon and this really creepy guy in a van picked me up, and the ride lasted about an hour, after the first few minutes of silence his first question was "so how old are you?" not, what's your name or where are you going lol. Then he was making all these weird noises the whole time, kinda like repressed sex-grunts, which actually kinda cracked me up, and his hand was placed on his crotch the whole time. There wasn't any fiddling, just a creepy fuckin place to have his hand. When he stopped for gas I just said i have a friend in this town im gonna go visit, thanks for the ride, peace.