Do not forget that being a woman you are firstly a human being and you have the same rights than males. You have a right to be respected and it is not true that while hitchhiking you expose yourself for a risk on your own wish.
For women, hitchhiking seems to be more risky. Hitchhiking as a woman can be easier though. Often, women stop for other women or girls. Even families (or other usual-not-hitch-hiker-friendly car owners) stop to "save" women from a situation in which they think she could be in "danger".
Some precaution you can take as a women hitchhiker
Whatever statistics are concerning violence to women, there are some things you can do in order to avoid/go out from difficult situations
Choose your driver
There are many different ways to get a ride from staying on the side of the road to asking directly to people at petrol stations. It could be better to ask people because you can then have a feeling of who is driving. It is of course difficult to know what kind of person someone could be just from how they look, but that first contact may compel you to avoid guys who already have macho or strange behavior towards you. If you want to or have to stay on the edge of the road, your attitude will of course attract different people. The 'positive attitude' thing is important but be aware of what you may suggest when standing and smiling. Body language is really important. I have a friend who almost got raped when hitchhiking in Europe for the first time. I hitched with her a few months later and understood then how this may have happened. She was standing on a half highway road without sign indicating her direction and with a huge smile, a slightly denuded t-shirt and a suggestive attitude. I am not saying we should all wear burka when hitchhiking, some people also use their charm and flirt a little bit with people without any problems. It is just a matter of being aware of how we do it.
Also the car sometimes tells about the owner. I am personally more careful when asking. For example, I flatly turned down a car with a big sound installation and loud music since this can show that the driver gives a lot of importance (and money) into appearance, and probably likes to be seen and to impress. It is not rare that their car is also a tool to flirt and impress girls. This is just an example and there is of course no generality. I have often met really nice people, which at first glance I wouldn't think of as sympathetic. Just follow your feeling and don't hesitate to refuse a ride if you don't feel like it!
Listen to your yourself / set up some safety rules
Set up some rules for yourself before begining to hitch hike. I personally have this one that if I happen to think/feel something is strange with somebody, I refuse the ride or ask to be dropped as soon as possible, even in the middle of nowhere. This is also valid for your security whith some drivers driving like crazy or really fast because they usually do so or because they want to show you how fast their Audy TT is. It may be better to spend an extra night in the middle of nowhere (which can actually be really nice) than begin raped or smashed in a car accident.
Finally take something with you to defend yourself in case of trouble. A knive can often be enough to dissuade your agressor. There are also pepper sprays you can find in some specialized shops, thought difficult to use in closed spaces. Take care, in some country it is illegal to use it against people or to use it at all... Having a mobile phone and knowing the important number (police, friends, ...) may be useful. You can also set up your own safety rules as for example always take your bag at your feet if it is not too big, in case you have to leave fast, etc...
Another tip if possible is to send the license plate number to a friend by text message. Always ask the driver first, if they object then don't get in their vehicle.