I store my homemade PBW in an airtight plastic container. Must be kept dry (and cool).
I put most bulk DME into quart ziplock freezer bags (Glad, zipper), ~1.5-2.0# a piece, used for starters. I also have a 5-10 pounds triple bagged lot, that's still dry and fine after 2 years.
i store my PBW in a big zip-lock bag.
DME is also in a big zip-lock, which is kept in a sealable bucket dedicated to DME. all is stored in my cool basement.