Also, try going over the marks with a dry erase marker. I know, it sounds crazy, but the way permanent markers work there's a bonding agent that makes the pigments stick to whatever surface it's written on. Dry erase markers use a bonding agent that is designed to wipe off and with sufficient application the old pigment will stick to the new bonding agent and wipe away. I may not have explained it well, but trust me, it works.
I usually use a brown dry erase marker, and mark until the original (inevitably black) markings are no longer visible. Then wipe away, and presto!