One thing I just did was fold all of my plastic shopping bags from Wal-Mart and other stores. I can't take credit for this idea ~ I actually read it somewhere here on the boards, but for the life of me I cannot find it.
The orginal poster said to fold your bag like a flag, but I just kept halving mine until I had a long narrow strip. Then I rolled it up from end to end and put them in bundles of 10 and secured each bundle with a rubber band. I love this idea because it saves so much space.
I have come to depend on these little bags for a lot of things, so if the you-know-what hits the fan, I figure it may be a while before I will get anymore, so I am going to add some to my preps.