Make a passive (air) cooling laptop stand. The basic principle is simply getting the laptop up off the ground (or off your lap) to facilitate natural air flow beneath the laptop. Many sell for ~$30 like the one shown in the images (and may be worth the money) but while you're deciding whether to buy one, why not make one yourself?
Step 1: Gather The Goods
Here's a picture of everything you'll need. Things not shown in the picture you might need: Sharpie and a cutting board (in the picture, just forgot to label it). I happened to have an obscene amount of dry-cleaner's clothes hangers, so that's what I decided to make it out of. Go figure. :P
Step 2: Getting Started: Crimping
Position two clothes hangers with hooks touching. Straighten each hook using pliers, then begin to wrap the metal around the opposite hanger (I used two pliers, one in each hand to easily bend the metal). They should both be firmly crimped to each other when finished. This step is probably the hardest.
Step 3: Adding Support
WARNING: Do not cut like shown in the pictures. My hands were positioned like that only for the purpose of taking pictures.
Now we'll add the support dowels running perpendicular to the initial set. Cut these new dowels down a bit to match the footprint of your laptop (e.g. my PowerBook is widescreen so i need a rectangle, not a square). Now, on each end of these new dowels, cut a 1" slit. Insert these supports on both sides.