For periodic cleaning of the inside, grab up a can or two of compressed air and a very soft brush, something akin to a woman's blush application brush as far as softness. Always brush or blow air across and down, so that the dust particles move towards the base of the case. Use a damp rag to remove anything that won't vacate with a hit of air of the case bottom.

You can literally wash a keyboard under warm water and sponge ... but you won't be able to use it til it thoroughly dries -- might take a day or two turned upside down on a drain pan. I usually have 2 boards, one to swap in while another gets cleaned up and dries. Some say you can expidite that with a hair dryer but I've never tried it.