How can I make a better whiteboard cleaning solution?
Andselisk's recommendations are great, but you'll save yourself a lot of trouble if you just throw all the erasers out. They get crud on them quickly and after a while, they put more back on the board than they wipe off.

A microfiber cloth is much more effective, and works for a shockingly long time. I would bet on a filthy cloth over a brand new eraser, and unlike erasers, they can be thrown in the wash. You can get them specifically for whiteboards or just stop by an auto parts store.