My laptop overheats easily; how can I reduce the frequency of this happening?

I have an iBook G4, 1 1/2 years old, that doesn't seem to be able to run any slightly taxing programs for more than ten, twenty minutes at a time. I can't play any computer games, whether they're designed for laptops or not, on it without it overheating and making disconcerting whirring sounds. Consequently, my laptop is useful only for internet and word processing. I'd like to be able to use my laptop to its fullest extent without having to constantly letting it rest and cool down, so how can I reduce the frequency of its overheating spells?