How can I erase Dry Erase marker off of something?

So, I managed to get a sign, formerly pointing out an event, but which I thought I could put to other uses. There was a wooden post, a flat wooden board, and then the part which was flat, and felt to the touch kind of like a dry-erase board. The back side was even white, so I wrote on it, and it came off with a little effort when I tried erasing. Problem was, when I let the marker stay on for a little while, it stops coming off so easily. I guess it was some sort of plastic. Anyways, I need to get the Dry erase off of it so I can use it again, does anyone know of something that can do that?