You can mix breast milk expressed at different times together, viksam. But it ideally needs to be at the same temperature at the time of mixing - i.e. both batches chilled in the fridge before mixing or both warmed to room temperature.
When you mix, the age of the entire batch becomes the date of the oldest milk for purposes of when to use it by.
I found this page on kellymom.com pretty helpful. :)
I always resterilise the stuff in my microwave steriliser every time I take something out. But in my case I normally take out the stuff to pump and my DH grabs the stuff to make up a bottle from the expressed milk in the fridge, so we end up using pretty much everything at once.
I have the Avent bottle warmer, DesperateHousewife, so warm the expressed milk in that. But you could just run the bottle under the hot tap or sit it in a bowl of hot water for a bit. :)
DH and I got the bottle warmer mostly because we don't trust ourselves to get the temperature right, especially in the early hours of the morning. 
We do always test the bottles still before giving them, to make sure they're not boiling hot or anything.
Those who are expressing, how much are you getting each time? Assuming I've had around 3 hours to 'refill', I get 4-5 fl oz. But I get noticeably more on one side than the other... 
Is this normal? I feed pretty evenly.