Have you ever had a center-pull ball of yarn slowly slide apart while you're using it to make the ultimate project?
Save those net bags that you get fruits and vegetables in from the grocery store. They are useful for containing that ball of yarn with a mind of its own.
Take any labels off the bag and weave a piece of leftover yarn through the netting about an inch from the top. Carefully insert your rebellious ball of yarn with the end coming out the opening and pull the opening closed. Tie a bow and you're ready to go.