I don't drink buttermilk, but I do keep this as a staple.
When I buy a carton, I store the unused milk into storage bags in 1 cup portions. I mark the contents and measure with a Sharpie. Lay the bags flat in the freezer and then stack when frozen.
I try to remember to move the bag from the freezer to the fridge the night before. If I'm not that organized they can be thawed with cool running water or a cool water bath for immediate use.