I know it's off season for most people, but I'm still trying to get caught up with what I've already done so I can show you what I'm currently doing. I tried this nifty little trick last spring, and saved myself about $100 a month on average in electricity bills.
First you get a bunch of cheap mylar space blankets, a pair of scissors, and a roll of regular cellophane tape. Cut the space blankets to size with the scissors, and then stick them to the inside of your windows with the tape. That's all there is to it. It'll look like a mirror from the outside, to keep the heat out, and like dark tint from the inside to let some light in. Better and more durable than sticking aluminum foil to your windows.