Buy seasonal: As tempting as it is to have fresh roma tomatoes in February, the truth of it is, they're just not meant to be. Food has traditionally been seasonal: Apples, squash, cabbage, canned and root vegetables in the fall and winter, and fresh tomatoes, peas, lettuce, and other green veggies and fruit in the spring and summer. That's not to say that you can't indulge yourself from time to time, but when you buy those strawberries ripened in a refrigerated shipping container as they made the trip from Columbia, remember that they don't have nearly the vitamins, antioxidants, or tastiness that their summer cousins have.
Buy local: Check with your grocers or local farmer's market to see which of their stock is grown closest to you, and which is freshest. Unfortunately, even farmers markets can be stocked with stuff from hundreds of miles away if it's a highly desirable item. Check out these options for locating farmers markets in the US and abroad:
The USDA's Listing of Markets is a good place to start
Localharvest, a site with a great zoomable, searchable map of local markets
London Farmers Markets is a good resource for those living there
Or try a Google search for markets in your locale
Ask Around: People who know about markets usually know about freshness. Ask your foodie friends where they got the ripest, the freshest, the localest produce in your area, and you probably won't be disappointed.
::The Produce Riddle Part 1: Organic VS Local [by DM]