Homemade waist stretcher. Just mod a bar clamp. I got mine for about $11 at my local hardware store.
It's been a life saver buying online. Just button up, stretch gently to desired width after washing (YMMV - depending on how good your hardware is. I haven't had a button break yet, but then again, I don't buy jeans with tin buttons). Let air dry. I follow the earlier advice and just purchase to thigh size (which you can't stretch out). I've had no problem stretching waist measurements to at least pre-soak widths.
One caveat. Don't use this trick to size down on a straight fit in an attempt to make it a slim straight. Patterns are graded for a reason. If you want a slim straight jean, buy the right cut (ie. Don't cop a cheap pair in the wrong cut during a sale thinking you can just stretch the waist. Won't work, and now there's a ruined pair of KMW 2010 that's probably still hanging in a thrift store somewhere...)