The best way for me to keep my callouses in shape is to play my acoustic. I use Elixir lights on it. The best tone I have ever gotten from it. I used to use DAdarrio acoustic mediums till i discovered Elixir :-)

(By the way it's a Yamaha acoustic I got from a Pawnshop a few years ago. I replaced the plastic bridge and the other thingy(I have a mind blank at the moment...can't remember the name) with bone ones. Huge difference in the tone)