Polyester fabric is naturally stretchy, but it is made to go back to its original form. This can make it challenging if you are attempting to stretch it out. It will be easier to stretch if the polyester is blended with cotton or another fabric. Use different methods, depending on how much you need to stretch the fabric.
Wash your polyester garment on high heat for at least one full 35-minute cycle. This method is best if you are only trying to stretch your fabric a tiny bit.
Put the garment on while it's still wet and wear it around the house. Allowing it to dry on your body may help it form to your body, thus stretching it out just enough to fit.
If the fabric stretched out enough, then continue washing it under high heat and allowing it to air dry.
Start by washing your fabric in a high heat cycle for at least 35 minutes. This method is best if you are trying to drastically stretch out the fabric, like a sleeve that is too short.
Remove fabric while it is still hot, and place it on a flat surface.
Secure one side of the fabric with something heavy, like a free weight or a few heavy books. Stretch the fabric as much as possible without tearing it. Secure the other side of your fabric with another heavy item.
Allow fabric to dry. Let it sit for an hour after it's dry. Remove the weight from the fabric and see if it is stretched to your liking. If not, repeat the process.
If you fear you've overstretched your fabric, wash and dry it in high heat. This should shrink it up. Try to avoid shrinking and stretching any fabric too much, as this will lead to permanent damage. If your garment still is not to your liking, you may need to make a trip to your local seamstress or tailor to have your garment professionally altered.