I had this problem also. I switched over to Wrangler Q-Baby jeans for riding and they are fantastic - However they stretch a bit. But no wear.
My new love though, is Tuk Tuk pants. They make them out of this fantastic material that is so stretchy - In heaps of colours including denim, and you can get different height waist, leg cuts, suede knee patches, suede full seat, fringe - Basically I think they are the bees knees plus they are super flattering. I have one pair and am about to buy two more.