With this one simple trick, you will never need to worry about that again. To fix this, sew a small piece of elastic to the back of your jeans
.Website I Am Momma explains how to fix this with a small piece of elastic and some sewing skills.
What you need is a scrap of elastic, the wider, the tighter. Sew it on to the backside from the middle of the seam with zig zag stitch.
Pull the elastic as tight as you can during sewing, and the elastic will pull the back inward tight.
This simple trick will forever fix your pesky jean-gap problem, and you will never need to worry about someone being able to see down your pants again! As you can see the elastic creates a small bunch in the back of your jeans, eliminating the gap altogether. Second chance by Susan also has a similar idea to sew in elastic to make no more gap.