Your knees and elbows are the hardest points on your body, so if you are in a position to use them, do so. Also, to steal from Miss Congeniality, SING. Solar plexus, in-step, nose, groin. And raking the side of your foot down the attacker's shin usually buys you enough time to break free and make a run for it. Eyes are also a vulnerable point. Scratching the face may not be the thing that makes them let go, but it does provide an easy identification marker for police; makes them stand out in a crowd.
Never underestimate the power of mace and an air horn, or the fact that the more noise you make and the more of a fight you put up, the less likely the attacker will follow through. Many of them are cowards looking for the easiest prey they can find and can be deterred. But always remember, never press your advantage! If you have the chance to run for it, take it! Don't go back and try and beat the living daylights out of the guy.