buying organic
"some fruits and vegetables consistently carry much higher levels of pesticide residue than others": "Apples, bell peppers, celery, cherries, imported grapes, nectarines, peaches, pears, potatoes, red raspberries, spinach, and strawberries."
ConsumerReports.org - when buying organic makes a difference (via kottke).
more detailed info on pesticide levels on different fruits and vegetables.