Cleanliness is next to ....

I have worked on a few desktop system recently that had one major thing in common.

  They were filthy!
The above part is the base of a heatsink-fan combination, which helps to pull heat from a component (in this case the processor) and keep it from overheating.  (imagine a small fan mounted on top of this part to help move air).  All this dust is kind of like a layer of insulation, and keeps air (and heat) from moving freely.
Cases can also suffer from dust infiltration. A standard PC tower may have as many as 4-5 fans, but it may only have 1-2 intake and exhaust ports to move air through the tower. (Laptops are typically worse, they may only have one main air path, which is intended to cool the entire unit.  A block along this path will dramatically reduce any cooling effort, and temperatures will rise..

Falcon Northwest is well-known for making excellent computers, mostly in the high-end gaming niche, and they demand top quality designs for their PC's because they are frequently run at higher than stock speeds (over-clocked)
So, how do you keep it clean?