I mount my mmm's sideways in the hope that the sand has a chance to drain out the vent on the underside (instead of building up around the board). I hear other blokes seal their motors with silicon to try and save the rotor wrapping and bearings. Obviously, cv boots help. I also use bluetak to cover the spare holes in the reciever. Other than that I'm not sure what else
Sand gets in EVERYWHERE but if you can slow it down...