You can divide it into 1 cup containers and freeze it. Then when you need buttermilk just set it out on the counter to thaw. Or if your in a really big hurry you can thaw it in the microwave on the defrost setting.
I don't like buttermilk to drink so when I buy it, it is always because a recipe I am using calls for it. None of my recipes ever uses a whole half gallon of buttermilk so I use what I need and freeze the rest.
Edited to add: It will separate somewhat when it thaws, but this doesn't affect the performance or the taste of the recipe that you use it in.
All of the above and thanks for the freezing idea. I have been giving the extra to the chickens and they do love it but am a little concerned that they are to young for the extra calcium. I only give yogert rarely because of it. But NOW I can freeze it by the cupful and then food saver it!
I bought buttermilk to make pancakes with. I had it laying around one day when I was gonna make pizza and used it warmed slightly to start my yeast it. WOW was it good. I also use it to make fresh NAAN, you know the indian flatbread. Same thing for any muffin recipe that might use milk or sour cream or yoghurt, use some buttermilk instead. SOOOO good.