Keep it clean: While using a pump, be sure to clean all parts after each use. Doing this in addition to washing your hands before and after using the pump will prevent cross contamination.
Store carefully: Breast milk can be stored at room temperature for a maximum of 4 hours, after which it must be moved to the refrigerator. Here, it can stay fresh for up to 2 days. If you choose to freeze your milk, it can be used 3-6 months after expression if kept at subzero temperatures. With this being said, it is extremely important to label each sample with the date at which it was expressed and/or the expiration date. When choosing which milk to feed with, use the FIFO (first in first out) method.
Reheat safely: Frozen milk should be thawed in the fridge or gently brought to temperature in a bowl of warm water. Thawed milk can also be reheated in warm water, preventing hot spots and the destruction of important nutrients that microwaves are prone to.
Out with the old: Thawed and reheated milk should be consumed or thrown out within 24 hours. If unsure about the timing of a sample, it is better to waste the milk than to risk the child’s health.
This information and more can be found at: http://www.eatright.org/resource/homefoodsafety/safety-tips/food-poisoning/keeping-breast-milk-safe