Cleaning baseboard heaters is a fairly straightforward process, and it's one that should be undertaken at least twice each year — in the spring and in the fall. Keeping baseboard heaters clean is part of the maintenance required to ensure efficiency and performance of the heating element and other parts of the heater.
Be sure to turn off the power to the unit and unplug it, if possible, before cleaning. Remove the cover of the heater to expose the heating coils. Most are held on by a few screws or end caps that are lifted up and out. Use your vacuum's brush attachment to remove dust in the interior of the heater. Work gently, and use a small paint brush to dislodge dust and debris that are out of reach of the vacuum attachment. Clean the inside surface of the heater's cover, and then replace the cover.