How can I cool down my MacBook Pro?

Dear all, I have a MacBook Pro 15" Late 2008.

It used to reach high temperatures only when running heavy applications (e.g., video) or several heavy applications at a time. However, now it's getting very hot more frequently than usual (over 70 ºC and 5,500 rpm), so hot that it's annoying to use on my lap; the area around the touchpad feels also very hot sometimes.

This overheating happens even when I'm using only light applications, such as Word, Excel, R, iTunes (music) or similar, four or five apps at a time (including Skype and Adium in the background).

Since I bought this computer, I updated the OS from Leopard to Snow Leopard and then Lion. I also updated the HDD (750 mb now) and the RAM (8 gb now).

How could I avoid this overheating? Thanks for your attention.