Luckily I live in base so I was able to pick up a part time job tearing down, cleaning, and repairing power tools. The guys that run the company are super laid back so if I get called to fly, I just drop what I'm doing and go fly. It's not the most glamorous job but it keeps me busy and helps out the F/O pay.
While I'm on airport reserve, I find a nice quiet part of the concourse (as quiet as you can find anyway), put on the iPod, and catch up on reading. I also routinely go through my flash cards for all my limitations, action items, and whatnot. Sounds geeky but when something malfunctions or the ****** hits the fan, I want my actions to be automatic and not have to think about it. I owe that to the folks in the back.