I was taught a techninque where, while you guard your head, you use you elbows to grind into your opponents thighs. This was to make him relax his leg hold on you and thus enable you to pass the guard.
Though this worked on my, being the little girly man that i am, on my class mates that actually had more muscle and stuff then me, it did nothing.
What other ways are there?
Phill