Perhaps the most authoritative work on Building Online Communities is the book, Community Building on the Web: Secret Strategies for Successful Online Communities by Amy Jo Kim. The publisher, Peachpit Press, provides a compact version of her advice in this article on their website. Based on the conviction that communities online and offline share many of the same dynamics, Kim has developed the following 9 strategies and 3 underlying principles for creating and growing communities online:

Nine Strategies:

Define and articulate your PURPOSE
Understand why you are building it, who you are building it for, and what their needs are, and then communicate your vision.

Build flexible, extensible gathering PLACES
Create a small, flexible infrastructure of meeting places, that you will grow an adapt with your members.

Create meaningful and evolving member PROFILES
Profiles can help people develop relationships, foster trust, and provide the community with a context and history.

Design for a range of ROLES
Newcomers and experienced members have different needs, which can be addressed by providing different roles, with different levels of guidance, responsibility, and ownership, and by providing a system that helps members move up through these roles.

Develop a Strong LEADERSHIP program
Community leaders help drive the community by encouraging and teaching newcomers, and dealing with troublemakers.

Encourage appropriate ETIQUETTE
Develop ground rules, standards, and systems that foster constructive argument and keep conflict from spinning out of control

Promote cyclic EVENTS
Regular online events bring communities together, and provide focal points of participation.

Integrate the RITUALS of community life
By celebrating holidays marking important transitions, events, and rites of passage for members, you begin to create a community culture.

Facilitate member-run SUBGROUPS
Subgroups provide a way for members to carve out space for themselves and other members with similar needs and ideas

Three Underlying Principles:

Design for Growth and Change
Communities start out small and focus and grow organically. Dont over-design things upfront. Instead start simple and add features as they become needed.

Create and Maintain Feedback Loops
Members and Management must to be able to communicate effectively in order for the community to adapt and meet the needs of members

Empower your Members
Members need to feel like the community belongs to them, and that they are actively involved in creating, maintaining and growing  it.