As a long-time collector of shoes. I've found that many injuries resulting from participation in sports, especially those involving running, occur when using inappropriate footwear. Here are some helpful tips when looking to purchase new footwear. The newbalance.com site has the best explanation of what to look for, so I'll use some of thier info.
Things to take inot account are your foot size, arch type, and pronation. There are three types of pronation: under-pronation, neutral gait, and over-pronation(supination). Pronation refers to how your foot makes contact with the ground. There are many different inserts and solutions that can compensate for these conditions.

Picture from runnersworld.
The best time to shop for shoes is late afternoon or evening, as your foot swells throughout the day.
New Balance recommends bringing an old pair of shoes in so the salesperson can see the treadwear.
There are many different techonologies used in shoes that can be of benefit to some, but not all. Nike has Shox, Max Air and Zoom Air. Adidas now has small computers in the shoes. New Balance has the greatest selection of sizes and widths to accomodate shoebuyers with not so common feet.
If anyone has anything to add please do. There is so much more to take into consideration. We could kinda create a shoe buying guide.