I like what pimpsta said about the freezer bag, I never thought about that, and it seems like it would help being thicker and all. I have had problems trying to crush stuff using a bag and hammer before, the bag always ended up ripping...but the more I tried the best way I found is like this (other than getting a mortar and pester like dr rolls said. Look around here there was just a post about mortar and pestles and where to obtain, etc.)...
Put it in a bag (a freezer bag like pimpsta said), lightly tap it to break into smaller pieces. When you have a good amount of smaller pieces, I then use a credit card to go over it and break it smaller. If you keep using the hammer on it the bag will eventually rip and it will spill everywhere from the holes you made. So that's why it's good to lightly tap or tap just hard enough to the point where you can get smaller pieces to work with using the credit card.
Hope that helps somewhat...
-Steve