The only thing I can think of that hasn't been said, is rolling your thighs in. Only problem is you have to roll them in while keeping the rest of your leg in position. I don't know if It makes sense, but It works for me.

About posting without stirrups... If I can't get something in my lesson, my instructor says that if I don't get it next time, the next whole hour long lesson will be without stirrups. Needless to say, I've had many no stirrup lessons. I rather like it, though, because next time I ride my legs are as strong as can be.