Many people don't know that you can freeze butter. Real butter and "fake" butter (ie. Margarine). Stick butter and tub butter.
So when you find it on sale, buy it up and pop the extra in the freezer. When you are close to running out, just put it in the fridge to thaw. It takes about 24 hours. I tend to keep two sticks in the fridge at all times. When I use the last of one, I get out another. Simple as that!
This is frugal in that you can buy butter when it is on sale and it keeps well for about 3 months in the freezer so you don't have butter going rancid!