Whiteboard refreshing
In addition to the whiteboard marker trick, which may take several applications in the face of an excessively zealous board cleaning crew (we had one board that was cleaned so well it virtually glowed white), there's another trick that'll work. Actually, 'trick' is the wrong word to use, because this is the real way to fix it...
Whiteboard reconditioner is actually made to do this job. Now, it works best if it's used on a clean board, and if it's a really improperly cleaned board (such as our phosphorescent board mentioned above), it could take two coats. But it's generally a lot easier to use it than to use the marker trick, if you have an entire whiteboard which has a problem.
Note that if it's just old whiteboard marks or indelible marks, whiteboard cleaner (basically, bottles of the solvent used in the whiteboard markers) tends to work much better than whiteboard reconditioner (which seems somewhat similar to the solvent used in the whiteboard markers with a fair amount of silicone liquid added.) Of course, this isn't a particularly bofhish solution.
The bofhish solution is replacing all of the whiteboard markers with indelible markers, and relying on my secret stash of whiteboard cleaner and reconditioner to use the boards as intended...