classic vinyl mats, but those are wet-erase and I find them a pain in the ass to clean. I don't like having water or soggy paper towels around my pretty game books. I know a lot of folks, such as the folks who designed the Avenger (aka the Ultimate) Game Table use plexi-glass. However, that can be a bit expensive. For example, this site has a 18 inch x 24 piece for $9.00. You have to pony up some considerable coin if you want a good-sized map. The only bonus is that you can place any kind of grid you want underneath it (e.g., a hexmap, pre-printed maps, etc.).
I created my battle map from tileboard from Home Depot. It costs about $11.00 for a 4 ft. x 8 ft. sheet. It is a heck of a deal, although toting it home might be a problem. I crammed mine into the back of my Jeep Cherokee and that was an adventure to say the least. I had to cruise down the interstate hunched over like Grandma Betty so that the tileboard could fit in the car. In hindsight, I should have just asked an employee to cut the board into a couple manageable pieces.
To grid the map, I used a dry-wall T-square and an exacto knife. I have found no other way to ensure a permanent grid. Even industrial strength marker has come off. A dry-wall T-square is marked just like a ruler, so that greatly simplifies the process of scoring the 1 inch squares. Go slow, though. Since you are cutting into the board, any mistakes you make are going to be permanent.