My good friend Paul Bleicher marched into work one day, early in Phase Forward history 11 or more years ago, with whiteboard cleaning spray and car wax. Whiteboard cleaner made sense ... the whiteboards in our new office space were a mess; but car wax?
After cleaning off the whiteboard with the cleaning solution, Paul proceeded to polish the pristine whiteboard with the car wax. He left it to harden overnight, buffed it off in the morning and, voila, we had a almost-good-as-new whiteboard which dry erased just great.
The car wax was important because over time the surface of whiteboards deteriorates (often by using those cleaning solutions). The car wax provides a new dry-erase surface that is almost good as new. The worst case is that you will have to go through this same procedure two or three times a year.
Some places on the internet recommend variants on this process, and others warn against it. Your mileage may vary, but Paul and I thought it worked great.