I froze mine in the Medela bags (for that purpose). I think Swispers makes a similar product which is heaps cheaper but I never used them. I ended up having over 20 litres in the freezer at the end! (after 4 months - I had lots of milk!) The difference between these bags and the normal zip lock bags are that the Medela bags are sterilised.

As for defrosting. I used to defrost in the bags at room temperature or to speed up the process, put it into a bath of normal tap water to defrost.

I wouldn't defrost in hot or boiling water. Also, they say that if you heat breastmilk too hot it actually kills the goodness.

Hope this helps!

I'm sure there is more information on the ABA website.