That's a tough one!  Generally the only way to remove a majority of the warping/waviness is to apply lots of weight for a long time.  Usually the thinner the module and the less bend or wave the better the results.  For me this is best accomplished with two pieces of wood, or plates of steel if you have access to them, sandwiching the item between them, and then applying a load.  I usually try to use something that weighs at least a 100lbs.  The more weight the better, within reason of course (never tried driving a car onto it).  I've also used a stack of modules before but that is usually not as successful unless the item has only mild warping.  You can try using a humidifier or steam to relax the paper fiber also but this can result in worse results or other issues such as mold/mildew.  It can work in some circumstances though.  You would then apply the load to let it set.  I would experiment with this technique using something that you don't care about first.