I really wish I had been able to do this with my jeans when I was pregnant.  Alas I was not even willing to get close to a sewing machine at that time - and so, I suffered with a copper button jabbing into my stomach for most of my pregnancy.  However, Jessica of "Running With Scissors" was a lot smarter than I was and she figured out a way to alter her regular jeans and turn them into some really comfy maternity jeans with some 2" elastic, and some stretchy knit fabric.  YAHOO!  

This excellent tutorial should make many moomies-to-be much more comfortable: