We tend to use our cutting boards like crazy, not just for cutting but serving as well. We received a nice set of Martha Stewart bamboo cutting boards a few years back for our wedding and they have since become pretty beat up.
When preparing some cutting boards to sell at our first show for Midwest Woods, we realized we ought to employ the same advice we had been giving friends and family regarding care of the cutting boards when they were refusing to use them for cutting ("they look too cool"!). Simple sand (if desired), and re-oil the the cutting boards with vegetable oil. That easy! And they look like new.
To oil ours we used a large paint brush dipped in oil. Kiel brushed them and I used a rag to wipe down the excess. And there you have it! You may have been aware of this tip already, but for those (like us) that had no idea and were about to give up and make/buy new cutting boards, make sure to try this first!