I have no idea what it's like to hitchhike as a girl, but I've never had a bad experience with hitchhiking. I get picked up by family folk, kids, old folks, church goers (I got a wonderful ride from a Mormon the other day), generally good people. I usually hear people say "I don't usually pick up hitchhikers, but you look okay.". So maybe it's because I stay clean cut and not bad smelling, but I don't have scary problems. Hell even when I'm travelling through some bad neighborhoods people don't mess with me, in fact, I usually meet somebody who thinks I need protection and they'll escort me to better places. Those people are AWESOME! They'll see you walking with a pack on your back and a smile on your face and be like "what the hell are you doing HERE?"
I've met some girls who have also hitchhiked through some pretty rough areas and had no problems. I mean generally, there's sickos anywhere, shit you're worried? I could be worried too. There are guys who would take advantage of guy hitchhikers too you know? Hell somebody could make a happy meal out of me just because they think they're doing the world a service. Has that happened? Nope, so I don't worry about it, no point in manifesting it. That's why I keep a 3" folding blade in my pocket at all times (I've only needed it as a tool or eating utensil though).
Also, trust your instincts. People's nervous system runs on energy right? And energy can transfer between multiple conduits (people), right? So when somebody sketchy picks you up, their energy will show in their body language, it will show their intentions, it will show what they are preparing to do. If they intend to harm you, you'll probably notice it, just learn to recognize that instinct. I find this instinct to be more common outside of hitchhiking, walking past people on a sidewalk is wear you can analyze people to see what their energy flow looks like. You'll notice some people follow their heart (they stick their chest out), but they don't consider their well being (tummy tucked in). Some people feel vulnerable and close themselves off (slouch in order to protect vital organs with back, shoulders and arms). Some people are well balanced (good posture, proper, appropriately cautious, not afraid, good energy flow, chi aligned). When your instincts tell you not to trust somebody, it basically tells you to bail.
If you're concerned about defending yourself, a walking stick is a great prop for hitchhiking, you can use it to point at signs (big, green, city is this far away, signs) with your thumb out. You can decorate it with string and twine and bungy cords and cool stuff on the side of the road. I've been picked up 3 times this week because of my walking stick. Also, you can use a walking stick to set up a tarp as a tent with a bit of string and some tent stakes. Get comfortable with your walking stick if you think you might duke it out, go in the woods and get used to the balance of spinning it, the vibrations when bashing trees, throwing it as a spear at targets, have fun with it, get good at it, wide stance for balance, find a sparring partner. Then if somebody wants to mess you up, you at least feel a little more confident in your ability to defend yourself. If you don't think that's enough, like I said before, pocket knife in your pocket. Still more? Learn hand to hand. In some cities, you can find on craigslist free martial arts classes. You can always find bored security guards who will share what they know.
And all of that ^^^^ I would simply recommend you learn simply for peace of mind, because I doubt you'll really need to use it as much as you might imagine.
What states are you thinking you may travel through? What type of regions do you prefer (city, suburban, urban, national forests, industrial areas)? do you have any experience hitchhiking at all?
