Follow these guidelines from the American Dietetic Association:

Should one buy fruits or vegetables in season or out of season? At the supermarket or from the farmers' market? These are frequent questions that consumers often face when purchasing fresh produce. Most health risks that are linked to produce can be eliminated with proper food preparation like cleaning produce thoroughly. Below are tips from the American Dietetic Association to keep in mind when shopping for your next bunch of grapes or head of lettuce.

Buying

• If you go to a farmers market, go early to avoid produce that has been sitting out all day long
• Buy most produce in season when possible
• If you are not satisfied with the store's selection, ask the produce manager if there is more available
• Buy loose produce rather than packaged
  • You have more control over what you select
• Don't purchase produce with mold, bruises or cuts
• Buy only the amount of produce that you will use within a week
• Buy only pasteurized juices

Storing

• Promptly store produce that needs refrigeration
  • Fresh, whole produce such as bananas and potatoes don't need refrigeration
• Refrigerate fresh produce within two hours of peeling or cutting
  • Throw away leftover cut produce that is left at room temperature for more than two hours
• Discard cooked vegetables after 3 to 4 days

Preparing

• Wash all fresh fruits and vegetables with cool tap water immediately before eating
  • Scrub firm produce such as melons and cucumbers with a clean produce brush
• Remove and discard outer leaves of lettuce
• Use two separate cutting boards to avoid cross-contamination
  • Use one for raw meats and the other for fruits and vegetables
  • Color-coded cutting boards can help you remember which is which
• Cook raw sprouts (alfalfa, clover, etc.); it significantly reduces the risk of illness

Source: http://www.homefoodsafety.org/pages/tips/tips/produce_safe.jsp from the American Dietic Association