I took some 1/2 inch elastic and I cut a strip to be about 6 inches long. I sewed it on, but let me explain how.
I started at the middle and sewed to the end using a zigzag stitch. As I sewed I pulled the elastic quite tightly while I let the denim layer feed through the machine normally. Then I did the same thing again. I started in the middle and sewed to the other edge. This made it so that the elastic was centered and I didn’t stitch throught the belt loops on the other side. Lastly I did one stitch in the middle, also making sure not to stitch through the belt loops.
Now I don’t have to bulk up with a belt. You can’t really see the stitching, since I used navy thread. Best of all, now the Grand Canyon is the only canyon people are staring at.
EXTRA INFO: This works best with lighter-weight denim. I haven’t tried it on really heavy denim yet. To really get even more pull inward, you could try a wider elastic and sew two lines along it.