Law.com's Small Firm Business has a piece by Edward Poll called "Ten Ways Blogs Boost a Law Firm's Image." The lead paragraph is pretty much on the money, I'd say:
When done right, blogs are an informative, vibrant calling card that represents you to the world. When done wrong, blogs are a chore and unwelcome expense, and produce little return on your effort. The difference between the two depends on having a realistic understanding of what you face before you start a blog.
Poll's bullet points are:
1. Hit the target.
2. Demonstrate your expertise.
3. Write clearly and concisely.
4. Make the commitment.
5. Image counts.
6. Everything has a cost.
7. Do your homework.
9. Remember your professional responsibility.
10. Keep your perspective.