Well in the case of my laptop I just turn it off, turn it around, and literally vacuum the roster from which the cooling fans blow out of.

You'll have to err, "suck" real "hard", and it may take a little bit untill the dust starts "coming" loose, but eventually you'll see it appearing, and you can either suck it out of the roster of use your "fingers" and err, fiddle it out.

Of course you could also just unscrew the roster, but I'm too lazy to do that.