The easiest way is to stumble upon a popular villager and sell them for an absurd amount of bells. I prefer not to do this, though. I've only sold one villager for 1 million. I could've asked for more because she was popular, but it didn't feel right. -shrugs-
Perfect trees are good, too. You could try to find someone with your perfect native fruit as their premium in Retail. It's best when they don't have the same native fruit as you, of course. Keep a ton of fruit in your storage to really profit when the opportunity presents itself.