Can't get the dry erase marker completely off of your white board? Use a paper towel and a dime size amount of hand sanitizer. Repeat if needed until marker is gone, then allow to dry (which is almost instantly).
Source: My husband tried it one day and it worked and we've done it since.
Sounds logical, since the main ingredient in hand sanitizer is alcohol. And, whatever thickens it, probably makes it stay where it should be for longer.