1) Thinking things through, internal long-, and short term roadmap.
2) Create interesting unique content.
3) Realize that your moderators are and should be a team, a go to point for the community - helping with a smile rather than play police with a badge.
4) Don't control your audience or their content. Despite that the submitted content might become your property (the post), they're the author. You can at most have your community grow along with the content the user are creating, or funnel the data into a direction - rather than tell the audience what to post.
5) Use socializing, word of mouth is still the most important free and valuable advertising method.
6) Consider a community manager, a private partner, register as a company and find funding to get things done right.
7) Care. Patience and Dedication.
8) Repeat, revisit, reconsider and re-apply constantly.
And even then you might just have 1 active member posting with a smile. Sometime it takes a bit of luck.
By giving the community what they want, where it is practical. By dealing with negatives/trolls/spammers quickly and firmly. By recruiting a dedicated and supportive team of moderators, and by working very hard to keep it all running smoothly (from a visual as well as technical POV).