How do I manage multiple jobs?

So I'm trying to pitch in and help a friend build a small business. I trust in his idea and I know we're a great team. The only thing standing in the way is this- I have a very busy schedule and not that much free time. My job sucks up a lot of energy. But I really do believe in his concept and it has a tremendous amount of potential. How can I find a way to add this task into my life and still keep a sense of balance?