you, and a million other guys. Just say what comes naturally to you. Have confidence first. like wear an outfit you know you look good in, that's usually advice they give teen girls in magazines for how o flirt better. Don't worry, be yourself. If things get.. dry, just try making small talk. Tell her you like her shoes, or something. Give a genuine compliment.
Own the conversation, like I do :) Have an opinion, listen to hers. Make her laugh. Ask her about herself--that will keep things going for a good while, and she'll feel like you're listening. It's a win-win.
Have confidence, ask her questions about herself (a woman's favorite topic... no offense ladies) and be legitimately interested in what she has to say. DO NOT put yourself in the friend zone though... make your desires known, but don't be blunt, be playful and coy.