I should have paid more attention but after regular washing of top surface, my half-inch-thick board is rocking like a cradle.  It is not practical to use flipped over as it has a counter lip.  I tried keeping the backside moist over 24hrs with no noticeable affect.  Wet grass and sunshine was suggested here but it doesn't sound very practical for me. What indoor solutions can I try? 
Not much you can do about a warped wooden cutting board to be honest apart from either chuck it out or live with its warpedness! I do sympathise, I have a warped one too!  In future (I'm going to heed my own advice) use food grade wood oil on the wooden cutting board, this goes a long way to stopping it from warping, protecting the surface and making it easy to clean. 