The American Academy of Orthopaedic Surgeons offers these suggestions when trying on and buying shoes:

• Before you try on a new pair, have both of your feet measured at the store. Your feet can vary in size, and change over time.
• Stand up while your feet are measured, and do it at the end of the day, when feet are likely to be a bit larger.
• Make sure that your shoes aren't so tight that they pinch your toes, but they shouldn't be so big that they slide around on your feet.
• Before you buy, walk around in the shoes to be sure they are comfortable. You should not have to "break them in."
• Sizes vary between shoes and brands, so go with what fits, not necessarily the size you are used to buying.