It's a design flaw with the case, really. To put it in car friendly terms, you can't have LS-2 V8 engine and expect it to be air cooled.

Grab a can of compressed air and blow it out. You'll be surprised by the amount of crap that will get blown out. Do it to the exhaust first, then the intake. Doing it this way helps to loosen all the build up because of how it clings to the cooling fins so when you use the compressed air on the exhaust blowing it out the intake it might get a little chunky. Then hit it again the way air normally flows from the intake out the exhaust and you should be fine.

For laptops I do this once a week, and desktops as needed or once a month.