What techniques professional pilots use to keep their piloting skills sharp when not flying for an extended period? I don't get to fly as often as I'd like. I know that simulators are good for brushing up on technique, learning new systems, etc.

Are there techniques that pros use (commercial or military) to stay sharp about things when they aren't able to actually fly? Specifically things that I would do not in a simulator, but say I have a couple months where I can't fly, but have an hour commute, or 30 minutes per day that I could do something flying-related to stay sharp.

I realize there is no substitute for actual flying, but I always feel a little rusty (mostly I just feel a little behind the airplane) when I haven't flown for a month or two. Are there things that other pilots do that I could learn to do in my down time that would mitigate this?
Reading a lot is key; staying on top of your checklists too. I noticed I'm behind the airplane more depending on the complexity of the aircraft in question. I keep photocopies of my checklists at home to refresh my mind prior to a flight.

It should also be mentioned the value of sitting in your aircraft on the ground; it costs nothing, and you can take your time working through checklists and scenarios in your mind.