The CDC has an excellent chart detailing how long one can store breast milk in various environments. (Click to download The Academy of Breastfeeding Medicine protocol for human-milk-storage information.)
There are 3 main types of containers parents in the United States use to store breast milk—polyethylene bags, hard plastic bottles, and glass. Out of the three, glass is by far the best material in which to store breast milk. (Stainless steel is rarely used in the States mainly due to the inability to see through it.) The main benefit of glass is that the immune components of milk are best preserved in it . In comparison, when hard plastic is used, the milk components do not adhere together as well. Another major advantage of glass over plastic is that with glass there is no need to worry about BPA and other chemicals that may leach from the plastic and may contain estrogenic or even androgenic properties. On the other hand, although glass is strong, its greatest drawback is its weight and its breakability factor as opposed to plastic which is durable. However, plastic is prone to scratching due to frequent cleaning, which can lead to bacterial buildup. Thus, such plastic bottles are best used for short-term storage. Polyethylene bags make for an inexpensive choice, but there are just too many negatives to even justify their use.
While at least two dozen people have given me positioning and latch advice, I don't think anybody every told me how to store breast milk. At least, not until I had been doing it wrong for a while. And so, in the interest of hopefully helping someone out there who is similarly ignorant, I'm going to tell you a couple of things about breast milk storage.
Another tip in feeding stored breast milk to a child is that babies have sensitive taste buds. Thus, they will not drink milk that has gone bad. So if your baby is hungry but does not drink stored breast milk, as painful as it may be, just throw it away and use a new batch.