Freezing is notorious for drying things out. Hence the term Freezer burn although I don't think it applies with weed. The best way I can think of to prevent this is the trusty foodsaver vacuum packing thing. Only problem with that is the compression factor. But is should have exactly the same moisture content it had when you bagged it up. And the nice thing about those things is you can make the bag too big and when you take some out, can reseal the same bag over and over.