So you grab a pair of trousers and suddenly you need to or lay on the bed and use pliers to get the zipper up. After you finally get the trousers on, you suffer from the dreaded "muffin top"--when trousers are too tight around the waist, causing fat to bulge over the top of the waistband. Many people can relate to the problem of a little bit of weight gain suddenly sneaking up and making trousers a bit too snug. Instead of reinvesting in a whole new wardrobe or suffering in trousers that are too tight, there are a few things that can be done to stretch the waistband of your trousers.
Launder trousers as normal, but do not dry. Alternately you can thoroughly wet the waistband of the trousers with warm water.
Pull on the waistband of the trousers in opposing directions, alternating between pulling from front to back and side to side. Apply enough pressure to stretch the fabric, but do so carefully to avoid ripping the fabric or popping off buttons. Repeat a few dozen times. Place trousers on the ground. Step on the waistband with one foot and pull upward with your hands. Rotate the waistband under your foot so that you are stretching trousers evenly.
Put on the trousers. They should still be wet, so this might be a bit uncomfortable. They should already fit better due to all of the stretching. If not, take the trousers off and continue to manually pull on them. Once you have the trousers on, do some stretching. Bend at the knees, keeping the back straight (deep knee bends). Bend at the waist and touch your toes. Do some lunges. Continue a few times. Manually stretching the trousers while on your body will allow them to better conform to your body.
Continue to wear the trousers until they dry, going about normal activities. The longer you wear them, the more they will stretch. You can also take them off and lie them flat to dry. Putting trousers in the dryer will negate the stretching effect, so avoid doing it.
The trouser-stretching is not permanent, and they will most likely return to normal after being washed and dried. Repeat the process after each laundering as needed.