The trick to getting permanent marker off your whiteboard is simply to write over it in whiteboard marker!
As discussed in our previous post about what makes whiteboard markers erasable, markers are simply pigment carried in a resin and solvent. The solvents are typically alcohol based, the same that may be used in other cleaners that can remove permanent markers. The solvent in the whiteboard marker can re-activate the acrylic resin and can be carried by the oil-based resin of the dry-erase marker.
This tip works best on FRESH marker, the longer it is left the harder it will be to remove.