Keep it in a dirt-free place (as if such a place exists!!!). Cloth cover - essential. Wipe it once in a while - keys and everything. Don't use anything corosive to wipe it, and don't get water into it. And above all, keep it away from toddlers without your supervision! My son climbed the shelf the other day to get a book - he fell, knocked the keyboard and the keyboard stand broke into pieces. Good thing my son and the keyboard are not broken!
Also keep it away from direct sunlight (ie away from windows).