It's happened to me 3 times before (yes I can count them!) and ONLY by accident.
I've only recently regained a sexual drive, and I've taken to rubbing my clit while having sex. It helps...but never actually achieved an orgasm this way.
We've been fooling around without a condom also lately (we've never had sex without a condom for longer than 15 second really...in the 5 years we've been having sex!). If he doesn't have a condom on, I can really feel him and I get closer.
But...once you've found out how, give more info.