Clean off the outside air compressor with water. A garden hose will do the trick.
Keep all things away from the compressor so that it has a free flow of air.
It is important to have your ductwork professionally cleaned every 3-4 years. This will not only make you breathe a little better in your home, but it will also keep your home from being so dusty. This process can be particularly effective if you have hardwood floors, since carpet will usually grab and hide dust bunnies.