Well, my girlfriend told me that she had never had an orgasm from oral sex before. She just didn't like it.
I convinced her to let me try, and I asked her advice. It didn't take me long to figure out what she likes, when her legs press against your ears and she's screaming.. keep doing what you're doing.
Suffice to say, she told me she hadn't experienced an orgasm in her life before I gave her oral sex and after she came I could feel her spasms for 30 minutes. She was blind for that time too
So in conclusion my advice; a woman knows her body best, ask her for advice. It's tough I know, but she will thank you afterwards!