Some of these are just good sense, some may not be available in the future...
http://www.askaprepper.com/23-things-prepper-never-throw-away/I already do some these, like keeping water in 2 liter soda bottles. I could use the smaller bottles too, but they are fragile and sunlight will rapidly destroy them.
Paper Clips - I throw mine at a magnet on the side of my desktop computer. When they get too thick to stick, I move them to a box and start over. Better than Rubber Bands, because rubber decays fairly quickly.
White Plastic Milk Jugs - Not good for food, but make great pots and buckets with a handle. Again, Sunlight will destroy them over time.
Soap Slivers - Kept in a small jar on the sink with a little water to soft them. I get a whole bar of soap out of this about once a year.