First, you should always make sure to put any unused breast milk in the fridge within a half hour after your baby has stopped feeding. If you let the milk sit around longer than that, bacteria can start forming due to saliva that gets into the milk during feeding. Once you have put a bottle of breast milk in the fridge, the fat will begin to separate from the milk, so be sure to shake the container well before reheating.
To properly reheat the milk, warm some water on the stove and place the container of milk in the hot (but not boiling) water; never reheat breast milk in the microwave. Once you have reheated a partially used bottle of breast milk once, it is generally safe to reheat only one more time before discarding the unused portion.
However, use common sense—just like regular milk, if breast milk has gone bad you will be able to smell it! Just remember that each time you reheat a bottle of breast milk, it will lose some of its immunologic properties, so whenever possible try to pump only the amount that your baby can eat.