Growing up my parents would regularly switch back and forth between buying organic and conventional produce. When I went to college I bought conventional produce because of the cheaper price and also the lack of understanding the differences. I've started to come up with a system (that is always evolving) as to what produce to buy in the organic form and which to buy in the conventional form. If I could I would always get everything in organic but that isn't always possible because of availability and/or price. Our grocery store has a decent Organic Produce section but sometimes I can't convince myself to pay over $4 for a single red pepper. From what I've seen there isn't as much interest in organic produce as there is in other parts of the country.
So to get back to the system I use...
The EWG, Environmental Working Group, has the mission of using the power of public information to protect public health and the environment. Every year they publish a list called The Dirty Dozen, which is accompanied by The Clean Fifteen.