[Sin Secret]

I just posted this answer to a very similar question, so I'll just copy it over here.

I totally feel your problem.
Unfortunately, the number one thing I'd advise for a quick fix is a vibrator.
I've been there (am there) and while I'm constantly searching for new ideas to help bring out my orgasms, the number one thing that has helped me is vibrators.

A simple, low cost vibrator (bought at a store or online at places like AdamEve.com or EdenFantasys, or anywhere else!) can (and probably will) do wonders for her. And you can get some of them for as little as $8.

Experiment with a simple discreet vibrator while you're fingering or going down on her. If you're having sex, have her hold it over her clitoris to stimulate it during intercourse. For me, that's where it pays off.
After some communication and experimentation, a well used vibrator can do wonders.

Encourage her to masturbate as well, especially with the vibrator, so she can get used to her own body, how it feels, what works, what doesn't, etc.

I'd also recommend this book I'm currently reading- The Orgasm Loop. I just started reading it and it's already begun to help.

Good luck, don't give up!

~Sin