DIAGNOSIS You're right to be careful. The main enemy is fine dust getting into the variable speed dial, trigger, and the motor vents. The goal is to get the bulk off without forcing debris deeper inside.
FIX Remove the battery. Use a stiff, dry paintbrush or a small shop brush to knock off all the loose dust. Focus on the air intake vents near the rear, the dial, and the blade clamp area. Then, use compressed air (from a can or compressor) to blow out the crevices. Hold the tool so you're blowing dust *out* of the vents, not into them. Avoid high-pressure air directly into the speed dial.
IF THAT DIDN'T WORK For caked-on grime, you can slightly dampen a cloth with water or a mild cleaner (like Simple Green) and wipe down the plastic housing, but keep all liquid away from openings, the dial, and the trigger. Never submerge any part of it.
HEADS UP Never use solvents like brake cleaner or gasoline on the plastic housing—they can craze or melt it. And definitely don't try to rinse it under a tap or hose. If dust is really packed in the mechanism, it's better to take it to a service center for a proper cleaning than to risk water damage.