1. Hire a passionate leader. A community manager who is passionate about the subject is critical (if not highly correlated) to the success of the community. If this is not in place, it is nearly impossible to succeed.
2. Focus on base camp at first, not the summit. When you first launch a community, spend your time and energy on your early, dedicated members. This is not the time for explosive growth (nor do you want it). Build up the credibility of the community slowly but surely to assure solid roots.
3. Tackle a specific niche . When deciding what your community will focus on, don't try to be all things to all people. Find a specific niche, population, industry or topic and keep it to that. Those who join will be much more likely to really care, converse, and the heat will be turned up.
4. Engage as a human, not as a monolithic robot. When communicating via email, newsletter, tweets, blog posts, etc, use members' first names where possible (and better yet, call out specific members for specific individual achievements, etc), mention something specific to them, celebrate community accomplishments, and generally be a real person with a reason voice, real concerns, and real imperfections...
5. Ask members specifically for their input. Upon first joining a new community, it can be intimidating. While some are comfortable jumping right in, some engage better when they are encouraged more. So go right out and ask members for their feedback and ideas. This will help move them along from just dipping their toe in the water to being an active participant.