First and foremost never store them in plastic... it is best for the fabric to "breath". If it is something you don't use regularly, roll don't fold with acid free tissue paper and place in a cotton bag, in a drawer or linen closet where the temperature and humidity is mild and constant.
They must be clean before storing and this is my go to mixture....
To a sink, or tub of Hot Water add equal parts of Dove Dishwashing Liquid and Clorox 2 Powder....and SOAK, Soak, Soak.... The heat and soaking is the trick. When rinsing be sure to remove all the soap. Repeating this process as many times as needed. If there seem to be some stubborn stains after the washing process the SUN is the best bleach you can find. I will sometimes use Baking Soda and white Vinegar on a very stubborn stain.
Every spring, my favorite thing to do is wash my linens,dresses,and quilts and air dry them. Even if you don't wash them, they at the very least should be aired and re-rolled.