One more point of note, don't use RED markers. They stain faster than other colors. I would stick to black and blue.
I also use Vis-a-Vis overhead markers (wet erase), which you should be able to get at an office supply store. Barring that, most game stores sell sets of Staedtler wet erase pens. Frankly, I prefer the Vis-a-Vis. Cheaper, and a better quality felt nib.
Wulf