Do you have any special foot issues? If so, you'll get the best service -- and the best selection -- at a specialty running store. Try on a wide variety of pairs in all price ranges, and base your decision off which shoes fit you the best. Many stores even have tracks around the store that you can actually run around to try them out.
What socks (or orthotics) do you use? Take them with you when you go shoe shopping. You'll need them to get a true feel for any shoe's fit.
Do you still have your old shoes? Take them along too. Trained, knowledgeable employees can examine the wear patterns to get an idea of your running style and gait.
Do you run every day? Consider buying two pairs of shoes and rotating their use. The theory is that each pair gets enough time between uses for the cushioning to fully decompress. Some users say that regularly rotating between pairs has greatly increased their shoes' mileage.
Avoid shopping for running shoes in the morning. It's a good idea to shop late in the day, after your feet have expanded from walking, since you'll want your shoes to fit comfortably even when your feet are swollen. Some experts recommend automatically going up a half-size from your street shoe size, since your feet may also swell a bit as you run.
Use your running shoes only for running. Mark your shoes or note the purchase date. Most running shoes lose their cushioning after 300 to 500 miles of use, depending partly on your weight and the shoe type. Even if a shoe does not look worn out, you may notice it feels less cushy or that you start to develop nagging injuries like shin splints. Keep track of your mileage and buy a new pair before problems start.
It should be noted that running shoe styles are often updated on a regular basis as companies strive to improve fit and comfort. New versions are typically released in two waves -- the first in January, the second in June/July. Previous shoe models are typically pulled from store shelves once the new version debuts, but can be found online at a discount until stock runs out.
Often, if a shoe line is selling well, the updates will contain the same basic design with just a few tweaks. Sometimes shoe models do change drastically from one model to the next, though. If you find you prefer an older running shoe to its newest updates, you might be able to stock up on extras before the older version leaves the market completely.