Second choice would be hard, clear plastic containers. Most moms find plastic is more convenient and some day-care centers will not accept glass because of the risk of breakage. All containers should have a tight sealing, one piece lid. Storing breastmilk in milk storage bags could present some problems. The milk could cling to the sides of the storage bag, reducing the amount that gets to baby.
Milk bags are also more prone to contamination through leakage.
Some pump companies make milk storage bags that are very convenient to use and are of a thicker gauge plastic than those originally tested, however, these can be expensive. If you do use bags, it's a good idea to double bag the thinner ones and store any bag in a hard plastic storage container with a lid, in the freezer. This will help reduce the risk of small tears in the bag. Ask your day-care provider that when they warm bagged milk, not to allow the water over the top of the bag, since this will likely cause water to enter the bag as it is opened. Colored baby bottles should not be used as some of the dyes may enter the milk.