A simple eyepiece cover is the most practical suggestion. These covers are normally used to block stray light from entering the viewfinder during long exposures, but they can also help keep dust off the rubber eyecup when the camera is stored or carried in a bag.

If dust buildup mainly happens in your bag, covering the eyepiece between uses is likely the easiest fix. Beyond that, regular cleaning is still the straightforward maintenance approach for rubber parts, since textured rubber tends to attract lint and dust.