One thing I do when I'm by myself, and have to walk to my car or home (especially at night), is to carry my car/house keys a particular way. I pick the key that is the longest, and I slide that through between my index and middle finger, and grip the rest of my keys in my fist. It gives you a bit of oomph (not too much extra but a tiny bit) behind a punch, and the shaft of the key can be used to poke at eyes, etc.
A couple of tricks my Marine cousins showed me - one is to bring your heel down as hard as you can across the top of the assailant's foot - lots of little bones in there and it hurts like hell when you stomp it for all you're worth - especially with heels on.
The other is that if you can't reach to scratch at their eyes or gouge them, is to go for the Columella (that leshy bit that separates your nostrils) - get ahold of it and pinch it as hard as possible. It hurts like the beejeebers. It's not much, but sometimes it's just enough to get someone to loosen their grip and you may be able to get free to scream and run.