I like my local running store a lot more than the national store in town. I have even gotten the local store to match the national stores prices.
Make sure you ask about their return policy.
I usually find a shoe that works for me and wear the same model for several pairs in a row.
Treat the sales person like you would a doctor. Tell them everything you know about your feet. Do you ever get blisters? Do you have pressure points in your feet? Do your feet ever fall asleep or go numb when you are exercising? Do you like a lot of padding? Maybe you want to upgrade the liner with a better quality one?
There is so much that you could possibly think about, but in the end it is all about how they work on your feet and that is where the return policy comes in, because ideally you will actually take them running before you decide if you are really going to keep them or not.
I go through shoes in 4-6 months depending on how much I am running in them. I don't stick to any set mileage when determining when a pair of shoes is finished. You just know it when they are. The tread is almost gone and they just feel dead.