It helps if you cheat .
Read a book on Boxing, or wrestling and do all the things it says are not allowed, such as groin kicking, eye gauging, and biting.
Whatever you do, do it all out, no hesitation and no holding back .
If you are one of those who doesn't want to hurt anybody then don't even bother, you have to approach self defense with the mindset that you intend to kill or cripple the SOB attacking you.