How are you defrosting the bags? Because your defrosting method may be the problem. How much are you putting in the bag in the first place? Too much milk and when it freezes, the bag ruptures.
Put the frozen bag in a cup and let it sit on your counter to defrost at room temperature. I know its not the quickest method, but it works. Then pour the milk into a bottle. If the bag does leak, you'll only lose a little bit in the cup.
Check out kellymom.com for more BM storage info or the pumping mom board at babycenter.com.
Just a note on a couple of other responses to this request:
1. Don't microwave breastmilk--it doesn't matter how long. It creates hotspots and "nukes" the nutrients in the milk.
2. Don't throw out your "bad" or old milk. I hate thought of my milk going down the drain. When I rinse out my bottles, I use that water to feed my house and garden plants and I'll even give my plants any "bad" milk. You should see how my plants have grown! Also, if you have any frozen milk that you don't need, try contacting a milk bank to make a donation. With my 2nd daughter, I've been overproducing and am donating some of my extra milk to the Mother's Milk Bank in Ohio. I've got almost 400 ounces stored in my freezer and I've only been pumping here and there since April! The milk is then processed and shipped to hospitals for preemies and those in need. Check out http://www.hmbana.org/