I don't think enough has been said about jacqui.c's post where she says appear confident.

It is probably the simplest thing to do and the most effective.

Shoulders back, head up and stride confidently, just a little faster than most other people. If you don't look like a victim type then most potential attackers will look for easier prey.

Oh, and a little bit of a scowl won't hurt either, make them think "I don't want to get involved with THAT BITCH".