1.  To clean a disc, use a soft dry cloth (preferably one that produces no lint.)

2.  Clean bottom side of disc with streight strokes from the disc's center to the outer rim only.  NEVER clean a disc in a circular motion.

3.  For stubborn dirt, use a solution of water with a little bit of dish detergent, wring out the cloth, and clean the disc as explained above.  Use another dry cloth to dry the disc.  Don't ever use any cleaning liquid other than mild dish detergent or you might damage the disc.