It's tough. What helps me is to step back and look at the problem objectively. Think about the situation is if it were happening to strangers and you were a disinterested observer. How should the problem be resolved?

When I'm in a situation like this, it also helps to think of myself as an "ambassador of reality". Basically, reality is sending a message to the business people. It's feedback saying "your assumptions about the market were wrong" or "you didn't plan thoroughly enough" or whatever the situation is. Reality just happens to be sending this message through you to them. If you don't ask for your money back, you're not giving them the proper financial feedback that they need to learn to be better businesspeople. For their next venture, you will have helped them learn that if you don't pay people they won't just go away and forget about it. You don't have to be aggressive or threaten legal action, you just have to be persistent and not go away. :)

If you need some inspiration, go watch the film Gandhi. He was always civil and friendly, but very stubborn. :)