Be Entertaining and Interactive
The Internet has certain advantages that other mediums can't match, so use them. Music, games, humor, and contests are all easy to provide given today's technology, and are popular forms of entertainment. Include Blogs, podcasts and Discussion Boards to provide interactive content. Push your readers to be active.
How do you know what they are interested in? Ask them... use surveys; ask for their likes, wants, needs. Ask for opinions on your niche... ask for testimonials... ask for help defining your content. Give and take is valuable. Remember it's a community you are trying to build.