Show off a little of your knowledge or just offer an informed opinion on one of the areas in which you practice. This attracts organic traffic and increases your exposure in search results.
Maybe a blog post about your decision to strike out on your own again — what are you looking forward to? What will you miss about working for your former firm? Describe the kinds of things you might be blogging about and ask people to sign up for your mailing list, even if you have no immediate plan to publish any sort of newsletter.