Since this thread got resurrected, I might as well add a tip that wasn't mentioned before. For cleaning a whiteboard, if the marker is REALLY set on there (like when you haven't erased it in a while), you don't need to spend a lot of money on those little bottles of "whiteboard cleaner." They're mostly isopropyl alcohol, which you can buy really cheap at the drugstore or grocery store (the stuff you clean your cuts with). Even if you buy an inexpensive spray bottle to pour it into and make it easier to spritz onto the board where you need it, it'll cost less than buying the "whiteboard cleaner."
I've found that the whiteboard erasers really clog up too, especially if you're erasing a lot at a time (or if you really want to get the board clean by spraying it). The cheap, easy and effective solution? A roll of toilet paper. Tear off whatever you need to wipe off the board, and toss it when done. It keeps the dust from the markers from accumulating too (yes, they generate dust too, just not as much as chalk does).