I know I’ve mentioned this in times past, but it’s worth illustrating with an excellent example: After you have balled your yarn, you need to ball it a second time.
Most people mount their yarn on a swift in order to use a center-pull ball winder. No matter how gently or loosely you think you are winding the ball, you create tension because you are pulling yarn from the swift AND pulling the swift itself round and round. The swift acts as a yarn brake, slowing down the yarn. (A yarn brake is a wonderful tool most often used for machine-knitting to deliberately tension yarn, especially when creating an intarsia project on the machine.) This pulls on the yarn and increases the tension on the yarn as you wind it up, which can cause the yarn to loose its natural elasticity. This will affect the yarn as you knit, especially if you ball your yarn and leave it stored that way for a while.
After you pull the yarn from the swift into a center-pull ball, put the newly-formed ball on the floor and reball it again. The second time, the yarn is free and is not also pulling the weight of the swift. The second time around, the resulting ball will be looser and larger, and will thus not lose its elasticity in case you aren’t going to be working with it right away. Reballing the yarn this way lets you store the yarn ready for use without having it stretched under tension as it waits to be used.
The two balls above came from virtually identical hanks of Briggs & Little “Heritage”, 215 yards each, part of the wonderful collection I showed a few days ago. The one on the left is a first-balled example; the one on the right is a re-balled example. You can’t feel it by viewing the picture, but you can readily see the difference. The one on the left is firmer; the one on the right looks and feels softer and more inviting to work with.