How do you keep a computer cool?

Is an often asked question and was asked of me by Jenny of Future Art Canvas Prints.

Keeping your computer cool on a particularly warm day is important if you are going to be using it a lot.  It is important to keep the computer cool for many reasons. The main one being that if your computer gets too hot it will simply turn itself off to protect itself and you will lose any unsaved data.  It can also shorten the lifespan of your computer and possibly damage the parts if it remains too hot for too long.

So how do I keep my computer cool?