People join communities to learn. Your members are thirsty for knowledge – especially about your community's topic. Your community must provide that knowledge. You must raise the level of expertise your community has in that topic.
This is cyclical. Expertise attracts expertise. A skiing community that has the members who can offer the best skiing tips will attract more skiers – with better tips.
You need to start the cycle. You need to cultivate that expertise.
You need to find experts to interview, create recommended reading lists and put a topical question to 20 of your community's most knowledgable members.
Identify your own community's experts (you can't automate this). Differentiate between members that post a lot and members that post good ideas. Seek out other experts. Gently ease them into the community. Provide a platform for them to shine. Give them columns to write and responsiblity for forum categories in their field.
Don't let any expertise go to waste. Curate the key advice into separate pages. Crowdsource a basic FAQ. Publish eBooks on behalf of your community (for free). Seek out guest-posts on blogs for your top members to write. Write your community's view to topical news stories.
Your expertise should be a major promotional point. You want your community to be the single greatest source of online expertise in its subject matter. This attracts more members, higher levels of participation and increases the status of being a member.