Career Planning: What's business and what's personal?

How much of your personal life should you disclose at work? These days when so many people are informal and open, it's even more important to set boundaries. The damages from self-disclosure aren't always obvious or swift. Even when you have your own business, you have to consider what to share with clients and potential allies. Some people believe in "let it all hang out" and others believe in tight compartments. I'd like to hear from the audience: what are your challenges, recommendations and solutions?