If you have access to an air compressor use it on the air outlet ducts (where the air normally blows out from),this should normally force the dust out from the inlet vents, if you dont have access to a air compressor use canned air instead.
I would not recommend using an air compressor. You never know what kind of muck is in the air. Depending on the environment they are in and how much they are used, they can have some nasty stuff in them that could blow into your machine.
You have a point there dmroeder, some compressors don't have filter or debris traps to capture contaminents or water, I've used the professional ones at car body shops which come with the extra ancilleries.