This Black medium point (1.0mm) water-soluble overhead-projection pen works well with mats.
NOTE: Never use dry erase, permanent, or grease-pencil markers. The vinyl is too porous for the lines created from them to be able to be removed. If you have any doubt of the suitability of any particular marker, draw a small line on the border, let sit for awhile, and see how well it can be removed. Better to have a small permanent blemish on the edge of the mat than a permanent map drawn on the entirety of the mat!