RULE #1, buy a shoe that fits!!!!! Shoes do not have to cost hundreds of dollars in order to be comfortable they simply have to fit.
Every manufacturer sizes their shoes differently, so do not buy shoes solely based upon the number imprinted in the shoe.
Buy your shoes at the end of the day after your feet have had a chance to swell a bit. Wear or bring with you the sock you will be wearing in that shoe. If you are wearing a dress sock or panty hose and try on an athletic shoe when you would normally be wearing a thick athletic sock, you may not get a good feel for the fit of the shoe.
If your feet have a tendency to swell, your best bet is a lace-up or Velcro shoe. Slip-on shoes will not accommodate swelling.