Store your breastmilk in clean glass or hard BPA-free plastic bottles with tight-fitting lids. You can also use milk storage bags, which are made for freezing human milk. Do not use disposable bottle liners or other plastic bags to store breastmilk.
And freezing milk does cause some degree of separation among its components. Skim and low-fat milk freeze (thaw, actually) better than whole milk, as there is less separation. You can shake the thawed milk vigorously or beat it in an electric mixer, but it still will not have the same "mouth feel" as milk that has not been frozen. The dairy board and other milk groups suggest that you might prefer to use previously frozen milk in cooking and baking, and save never-frozen milk for drinking.
I freeze milk all the time. We don’t have kids yet so I just need milk for tea and can’t use a whole gallon before it goes sour. When I open a new bottle I pour some into 4 or 5 little pots and put them in the freezer. I can use the rest of the bottle before it goes sour then I take the pots one at a time from the freezer and put them in the fridge to thaw over night.
When I get a good deal on milk, or for some reason don’t want to have to make a trip to the store for a few days, I freeze my milk. The containers that they come in from the store have the indentations on the sides that will expand when the milk freezes. I don’t normally pour any out when I freeze it.