First up, you should always place your laptop on a hard surface. That helps the fans circulate in the laptop. If you have your laptop placed on a soft surface (e.g. a fur carpet) the circulation is not as good and it will tend to get warm very fast.

The next way to cool down your laptop is to use a laptop cooling stand. These days, you can buy laptop cooling stands quite cheaply and these contain fans which help to cool your laptop immensely. They also encourage better posture by forcing you to use your laptop more ergonomically.
 

A typical laptop cooling stand

The display screen on your laptop generates quite a bit of heat. You will want to turn down the brightness if you are in a dark place, so as to cut down the heat generation. Turning down the brightness also allows you to conserve battery power.

An obvious way to cool down your laptop is to move to a cooler place. If you're sitting in the hot sun typing on the laptop, the machine ain't gonna stay too cool, now is it? Move to an air-conditioned room or cafe and cool the machine down.

Most laptops these days have a 'power saver' mode which you should invoke if the laptop gets too hot. The power saving modes usually turn down the display brightness and spin down the hard drives if the machine is idle for a fixed period of time.
 

The Power Saver Mode in Windows Vista

If you open many applications on our laptop, these will generate heat as well. With more open applications, the laptop's CPU has to work harder, resulting in more heat generated. Hence, by closing down some un-needed apps, you will cool your laptop down.

USB ports are a well-known heat generator. The more USB devices you have connected to your laptop, the more heat it will generate. Plug out the un-needed USB devices and your machine will start to cool down.