Just use ammonia free glass cleaner sprayed on a rag that won't shed real bad & wipe the tops of the keys off for a quick clean. Although as most others recommended if they are really dirty taking the board home, removing the caps, & soaking them in warm water with a denture tab or some mild dish soap, then scrubbing them with a soft toothbrush or soft rag is a much better way to clean them. The best & easiest way is toss them in an ultrasonic cleaner with a denture tab or tiny bit of mild dish soap, although ultrasonic cleaners skyrocket in price with the size of the tub & a cheap one for cleaning jewelry will not hold a quarter of a GMK set in one go.