So the very best way of fixing the problem of is to prevent it in the first place.
Some of our favorite closet staples are from there).
Second, wash your jeans in cool water.

Third, line dry them.
Namely: wash them in the hottest water setting on your machine and use gentledetergentand fabric softener.
Then, in spite of all previous advice, toss them into the dryer on the hottest setting.

This should shrink your jeans back to a more fitted style.