Add adjustable elastic to pants. I've done this to lots of jeans for myself and family members. Some jeans are sold with the adjustable waist elastic and now I even salvage the elastic and buttons before throwing out an old pair of jeans and reuse the elastic in a current ill fitting pair to save on buying new elastic. Helpful hint: Before pushing elastic through the waistband from one hole to the other, safety pin the end. It's easier to push a hard object through.