Try this one...
Grab your opponent's neck with both of your hands (slightly douchey, but works) and push his head up. Swing your legs up and trap his head with your thighs (you might have to scoot your hips out slightly to get the right leverage) and then cross your ankles. When you trap his head with your legs you can simultaneously release your grip on his neck with your hands and move your arms down around his waist. Next, just use all your strength to do a sit-up, which will basically bounce his head off the mat if he doesn't post. Anyway, once you sit up as far as possible, immediately do a barrel roll to the side you feel he has less base on and end up on top. I hope that made sense. The whole technique is very quick.
I learned this from one of my Erik Paulson DVDs, but I can't remember which one. It's very effective.