Personally, I think it's important to get to know the person first. Not necessarily for a long time, as friend zoning can occur, but show her that you developed an interest in her, specifically. People who have asked me out without knowing much about me really haven't attracted much of my attention, honestly. It's sort of like, "Why do you like me, you don't even know me!" But I'm probably alone in that. I want to know that I'm giving my trust to someone who both likes me and I know I like back. *shrug*
Just be genuine, be direct, and if it works then great. If not, don't persist. Persistance after rejection tends to irk me, because I've already answered the question once and don't feel like having to do so again. It's hard enough saying no to someone once, really!