Wash your jeans or soak them in a bucket or tub of water. Pull them out of the water or washing machine. Pull and stretch your denim in strategic spots, such as at the waist if they are too tight around your belly. To stretch the length, take the waistband in one hand and the hems of the legs in the other and pull as hard as you can. Repeat this process several times to loosen the denim's fibers.
Get those tight jeans on in any manner possible. Slip a slim bottle of deodorant into your waistband at your hip. If you have room on the other hip, slide in another object, such as a a bottle of perfume or hairspray. Walk around, move about, sit on the floor and let the extra bottle stretch the waistband of your jeans. Try some squats to give your booty some breathing room.
Press a hot iron to your jeans. The heat from the iron will loosen the wrinkles and make the denim fibers more pliable. Put them on while they are warm and loose, and let them mold around your rear and legs.
Avoid drying your jeans the next time you wash them. The heat from the dryer can shrink the fabric. Let them air dry. Make sure you add fabric softener to your wash load, or they will be hard and stiff.