I put them in milk storage bags and then bag them up at the end of each month in gallon sized bags with dates marked!
Bottles that I pump into, as I have a few spares, write the date on the lid with washable marker, helps keeping track. Then bag them in milk storage bags to freeze or use in a bottle, milk bath, etc
Milk storage bags from Asda or boots can be kept in the fridge or freezer and write the date it was pumped and the amount