Your mattress needs a proper cleaning every few months to get rid of dust and dead skin cells, sweat and oils from your body. To do this remove and wash your sheets as usual.
Then clean your mattress, use a garment steamer and go over the top, sides and box spring thoroughly to kill any dust mites that may be hiding near the surface. Then use your vacuum’s upholstery tool and go over your mattress again. (This will remove the dust mites steaming has killed.) Remove any stains with a carpet and upholstery cleaner without over-wetting your mattress.