Recent pursuits

I've been doing my best to read and write more, and have incidentally gotten myself back into gaming along the way. So far it's been entertaining, and a fun challenge to try and figure out plots and storylines that are both accessible to the players in my D&D group, and are interesting to me. Recently I've started wanted to play around with recurring villains, especially clever, scheming ones, but I'm not entirely certain what tack to take with this new challenge. How clever does a scheming villain have to be? When creating a villainous plot, should it be planned down to the very last detail and contingency, and what sort of allowances should be made just to "wing it"? In short, how much more work do I have to do as a GM to run a scheming villain?

If any friends of more game-ish experience have any tips, I'd be very appreciative!