Get as much dust as possible out of the innards of the machine, including the power supply. Use all necessary electrocution precautions of course.

Consider a Kensington System Saver - a self powered fan in a box that sits in the well on top of the Mac. You could probably make something like that yourself that wouldn't be too ugly.

I've also considered using one of the MiniITX DC-DC power supplies. That way most of the power conversion (and thus heat) is done outside (the AC to 12V DC converter). As a side benefit, you end up with a Plus you can use in the car