Either don't put yourself in a situation where someone want to brain you from behind or were a helmet everywhere you go.

Never be out alone. Alternatively, always walk at a brisk pace, give corners and alleys a wide berth, and don't wear headphones. If you're walking quickly enough anyone catching up to you from behind should have to make quite a bit of noise to get in range, thus the lack of headphones.

However, all the advice in the world will do you no good without constant situational awareness. Here's a nice guide to the basics, but practicing it is up to you. http://www.stratfor.com/weekly/practical-guide-situational-awareness

There isn't a way to cover your ass 100% of the time. Everybody steps on their own dick, every once and a while.