I inherited my Mom's itty bitty waist and big ole booty. Needless to say, I have the hardest time finding jeans that fit right. They are either so tight in the legs I can't pull them up, or big enough to fit two of me in the waistband. So, I decided to take matters into my own hands.
I found the first belt loop on my waist band, if your problem isn't as bad as mine you could skip the first one and start on the side belt loop instead. On the inside of the waist band under the loop, I cut a hole between the top and bottom stitching only through the inside layer of fabric. I did this on both sides of the waistband.
Be careful NOT to cut the stitching or through the front layer of fabric!
This gives you access to the inside of the waist band.
I used 3/4" elastic, because it will remain tight inside the waist band. Measure your waistband to insure this will fit and make elastic sizing adjustments if necessary. I cut a piece long enough to fit once around my waist. You will trim it later, but this gave me enough to work with easily.
You will want to use a good quality denim thread.
Hook a large safety pin to the end of the elastic.
Fish the elastic through the hole you made in your waistband, by pushing and pulling the safety pin through.
I pushed the safety pin until it came out the hole on the other side of the waistband.
I then, safety pinned it in place on both holes.
I wanted a really strong stitch to hold my britches up. I made them long, but really tight and close together. Your machine might be different from mine, so adjust accordingly. I suggest a practice stitch on a scrap piece of denim.
I sewed behind the belt loop, so that the belt loop would hide the stitch, if I chose not to wear a belt. Be careful NOT to sew the belt loop shut!
My waist is quite a bit smaller than my booty, so there is a little bunching, but if you throw a belt on you can't even tell.
They feel so much more comfortable, and don't bunch up weird under my belt like they did before.