I do two keeper leagues with two sets of friends and co-workers (and a little overlap).
I used to also do two or three public leagues to test draft strategies and such, but I don't have the time. It's too bad, though, because I think these extra leagues keep you paying more attention. If your always filling different needs on different teams, you pay attention to more guys.
I've also noticed that my two teams have a lot of overlap (Utley, M-Cab, Carlos Lee, Corey Hart, Cain, McGowan). I don't even really have to check two teams now.
"All opinions are not equal. Some are a very great deal more robust, sophisticated and well supported in logic and argument than others."
— Douglas Adams (1952-2001)