We try to buy organic when possible, but sometimes it's not worth the steep difference in price. So, when picking what produce to buy conventionally, I think about the Dirty Dozen vs. Clean Fifteen list you see below. This one was put together by the EWG (which I like to reference for sunscreen and skincare items). I have pretty much committed this list to memory, so I don't have to pull it up every shopping trip.
A good rule of thumb is to think about how thick the peel is on the produce, and whether you'll be eating that. I don't worry so much about buying organic bananas, because that is a very thick skin that the pesticides would have to penetrate. But, if I'm eating something that doesn't need to be peeled, apples for example, I try to go for the organic option. They tend to be smaller, but I'm okay with that!
A frugal tip: I like to buy my bananas at Trader Joes because they are sold by the item, not the pound, which ends up being cheaper than somewhere like Publix or Kroger!