Straining and groaning into a pair of jeans isn't any fun, especially when it comes time to zip and button them up. However, you can stretch the waist of those jeans that almost fit. All it takes is a regular wash and some elbow grease.
Put your jeans through a regular wash cycle on cold.
Remove the jeans from the washer and hold them in front of you.
Insert your forearm into the waist of the jeans, your elbow on one side and your fist on the other. Stretch out your arm and bend again at the elbow.
Insert a plank of wood cut to the correct size into the waist of your jeans.
Let your jeans dry naturally by hanging them to dry on a line or a drying rack. If you are pressed for time, tumble them in a dryer on the lowest setting.
Hold one end of the waist of the jeans in one hand and pull with your other. You also can insert your foot into the waist of the jeans on one end and pull with your hand on the other. Be careful not to overextend the fabric.
If you don't have a plank of wood, you can pull the waist of the jeans over the back of a chair, so that it hugs the chair and the chair back stretches the waist. If you do it this way, you can leave the jeans there to dry, maximising the stretch. Buying jeans with a bit of Lycra in them will allow you to maximise the stretch. Check to see if jeans are pre-shrunk before buying them. If so, it is unlikely they will shrink much more. If the pockets seem deformed or scrunched up, chances are they are pre-washed, pre-shrunk jeans.