Try your trusty washing machine. Wet denim is more willing to stretch out than dry denim, so toss your jeans into the wash and then pull the waistband over the back of a wide chair. No chair wide enough? Bust out your inner carpenter and cut a piece of wood long enough to exert some stretching pressure on your waistband. When the jeans are dry, you should have some extra room for wiggling.
Embrace your seamstress skills. If you want a permanent fix, you can sew in wedges of new denim along the side seams of your jeans. Just measure your waist and the waist of your jeans and calculate the difference. Cut two wedges from new, matching denim so that the flat edge is half the length of the difference. Snip along the side seams, and sew in the new patches and voila.
Visit the wonderful world of elastic solutions. If you know your way around a needle and thread, sew a two- to three-inch portion of two-inch-wide elastic into the waistband behind the button hole and button for an instant elastic waistband. You can get the same effect with a wide rubber band. Just loop and knot it through the button hole and use that as your fastener when it's time to button. You can also buy a professional elastic waistband expander from any fabric store.
Leave it to the professionals. Tailors are skilled at making simple adjustments to your clothes, so if you really love these jeans, or if they cost more than your everyday pair, it's best to leave the snipping and sewing to someone who knows what he's doing.