Hey,
I also have had my problems with buying wrong shoes. Besides listening to the salesperson, here the lessons I have learned (you never know if you are getting the best salesperson):
- if you are heavy or run on concrete, get extra padding ($$)
- if you have flat feet or fallen arches, get arch support ($$)
- when trying on shoes, beware any friction or supersnugness since it just magnifies on long runs; don't be afraid to ask for lots of different sizes, even different widths (2E is the next size wider than normal)
- shop around if you can, especially in this stage where you haven't found the best shoe for you; different brands work better for different feet, not just in size but in shape
- you don't have to buy a pair of shoes at that visit to the store (I find it impossible to walk out without a pair of shoes, costing me hundreds of dollars in wrong shoes)
- use your shoes in dry conditions on pavement at first so that you have the best chance of returning them if they are bad
- once you find the right shoe, you'll probably keep getting the same model; keep a mileage log on shoes; don't wear the running shoes for any other activity

Thanks,
Dan