Take in the side seams
Take in the back seam
Add darts at the back waist
Move the button and waistband inward
Shorten the waistband and reattach it
Use elastic in the back waistband
Add a waist cincher or adjustable tab
Sew in a hidden elastic panel
Use safety pins or temporary clips
Belt the pants at the waist
Have a tailor alter the waistband
Wash and dry if shrinkage is appropriate for the fabric