1) Treat her like you treat girls that you know are interested in you but you aren't sure about. You ever notice how when a girl likes you and you like her back but not that much, they try even harder to get your attention? Well do that with the girls you really do like, except on purpose this time.
2) Be funny, playful, and tease her. Under no circumstances act like a puppy dog looking for a home. In other word be FUN to be around. Girls like guys who are fun and exciting to be around. PERIOD. You should always be talking about how much fun everything is, and the fun you’re going to have tonight and tomorrow. You should possess extreme optimism, which women LOVE.
3) If you have attractive female friends (and every single, straight guy should have at least one or more of these), use them to your advantage. Make sure you are with them when you are around her. If she asks if that's your girlfriend it's a good sign. Be very coy with your response. "We just hanging out" is sufficiently vague to drive her out of her mind. It could mean you're just friends, or it could mean you're shagging her brains out. Either way your attractiveness goes up. Women want guys with high social status...meaning they want a guy that is wanted by other women. Men have a competitive streak that they satisfy with sports. Men are women's sports. What women want most of all is to be with a man who is desired by other women, but in the end chooses her. It validates her self worth and makes her feel special.
4) Do not ask her on a date like this "So would you like to go out with me sometime"? or some such nonsense. Say something like, "I'm going to this awesome bar tonight, you should come along". The phrasing is that something cool is happening and you're going to be there, and it's cool because you're going to be there, and if she wants to be cool and have a good time, the place to be is where you are. This should be your mindset all the time. The place to be is wherever you are, and she's welcome to come along, but if not it's her loss, not yours. More importantly you actually have to BELIEVE this. Your attitude about yourself is far more important than what she thinks. Don't try to impress her. Make her impress YOU.
Women are attracted to fun, confident, slightly cocky (not arrogant, but playfully cocky), mysterious guys. They love a challenge. If you're too easy to catch, then you have lower value. Good looking women have choices mate. If you act like the average guy, you'll get nowhere. She should never know exactly where she stands with you, which is why you should absolutely flirt with attractive women in front of her by being playful and teasing and fun with them just like you are with her. If she likes you, it will drive her wild. If she doesn't, she won't react at all and you don't have a chance right now anyway.
Good luck!