When you have to open a can or package of something but only need to use a small amount, divide up the rest and pre-package it for next time.
For example, I generally use 1/4 C of canned pumpkin but there are 7- 1/4 C servings in a can. I measured out the rest of the can in 1/4 C servings and froze them on a cookie sheet covered w/ wax paper. When frozen I put them in marked baggies and put them back in the freezer. Next time I can just grab a baggie and go without wasting anything.