Two ways that are recommended to store your muslin background is (1) to fold the muslin like a bedsheet and store in a cool dry place, or (2) go to your local hardware store and purchase a PVC pipe about 1"-2" in diameter and the same length of your background. (ex: your background is 10' wide, you will need a 10' pipe) Run the PVC through the Hem Loop (or rod pocket) at the top of the background. Roll your background around the pipe and store standing up or laying on a shelf.
The second method will help reduce the amount of wrinkling you will have and will speed up your steaming process the next time you pull it out to use it.
It is not recommended that you crinkle or smash the muslin into a muslin bag as this will damage the paint and could cause (over time) cracking in your background. If storing in a muslin bag, neatly fold the muslin to size and then insert into bag.