I learned the value of a five gallon bucket while working as a community garden organizer in AmeriCorps. We were the epitome of thrift as we'd try to scrounge every free resource we could get our hands on. We used five gallon buckets to haul soil and mulch around in the gardens because we didn't have a lot of wheel barrows. Restaurants and stores can be very generous in giving these to you if you can pick them up when it's convenient for them. I've also gotten many smaller two gallon buckets from my Costco that were used for icing in their bakery department. I use these buckets for my glazes, reclaiming clay, rinse water (to keep clay from going down the sink) and even mopping the floor.
Other plastic I reuse comes from packaging of items I buy. I don't know when it became necessary to encase everything in plastic (I mean really, why does a vacuum cleaner need a plastic bag around it inside a box??) but when I get the long, lightweight kind, I save it and use it to cover my pieces from drying out too quickly. One piece was long enough to be a complete cover for a bookcase that I turned into a damp closet.