This profile on Paul Polak by Cheryl Heller has some excellent advice. I highly recommend the whole article but I think this tip is especially important.
“5. Think huge, and don’t be a victim of your emotions
Paul’s rule is that a business has to have the potential to reach 100 million people and generate at least $10 billion in sales in order to be worthwhile. Seeing that potential will make it real.
While passion and empathy draw people to help others they are anything but the secret to success. Hard-headed business strategy will go much further to change lives. Caring deeply about helping people should spur pragmatism, not romanticism.
There are practical lessons here for all involved: Don’t fall in love with your altruism when you don’t have a sustainable solution to poverty, and don’t fall in love with your new business idea unless it can really impact the world.”