Green Geek has a post which asserts that by avoiding the 12 most contaminated fruits and vegetables (or at least buying them organic) and eating more of the 12 least contaminated, we can reduce our pesticide exposure by 90%.
12 Most contaminated: apples, bell peppers, celery, cherries, imported grapes, nectarines, peaches, pears, potatoes, red rasberries, spinach, and strawberries.
12 Least contaminated: asparagus, avocados, bananas, broccoli, cauliflower, corn, kiwis, mangos, onions, papayas, pineapples, and peas.