An external cooling fan certainly could help.
More importantly, you want to keep the computer on a hard stable surface. Don't use it just on your lap, on your bed, a blanket, a pillow (I think you get the idea!) without something hard in between--like a book or a binder.
Apple computers tend to run a bit toasty, and I've really noticed this with my MacBook. If you're doing anything that involves Flash (streaming video, some games), more of your processor will be in use, which creates heat, which will make the fan kick in. There's not a whole lot to be done about that, aside from Adobe fixing things so Flash doesn't inhale resources like a child does Halloween candy.
As for which cooling fan to use, I had a Targus Chill Mat, and it shorted out within a few months. I have this one:
http://www.amazon.com/Belkin-F5L001-Laptop-Cooling-Pad/dp/B000NWIOM6/ref=sr11?ie=UTF8&s=electronics&qid=1254328489&sr=8-1
And I really like it, though I don't use it with my MacBook. I use it with an iBook and an older Toshiba notebook which have troubles keeping themselves cool.
~Lyssa