I had a similar setup at a small company a few years ago. We first "primed" the surface by intense scrubbing with that Expo cleaning. This made it easier to wipe later on.

We also used Tabasco sauce for tough marker residue.

It's worth experimenting with different brands of markers and different colors to see what ends up easier to clean if left around for any length of time. If you really want something lasting, take a picture.

(I have a laptop-sized whiteboard I use like that: sketch out stuff, if it looks good, snap a picture to save it.)