I found this very useful. Thought I would share.

Is there a comma before but? The correct answers are yes there is and no there isn’t. But you need to decide. It is easy to make the correct decision. It will depend on the structure of your complete sentences. Luckily, the comma rule to help you decide is super easy to learn, use and…

via   When Do You Use A Comma Before Or After But? — Just Publishing Advice For Writers and Authors