So freezing my milk and buttermilk in one cup sizes has been a godsend. I'm still wasting more milk than I'd like, but at least I'm not using a 1/4 cup of buttermilk and throwing the rest of the quart away when it goes bad.

Just take it out of the freezer and into the fridge the night before I need it, shake it every so often as it thaws and it's good as new.