A sustainable community to me looks like one where the people know and help each other. This would make the sharing economy strong, the people more resilient to individual problems, and everyone more likely to contribute to community-improving projects, such as social and environmental advocacy, building community gardens, and keeping the neighborhood clean and safe.
For my area, a city, what first needs to be changed is the isolation between neighbors. That's not to say all neighbors want to get to know each other - and that's fine, of course - but I think at least half would like to get to know each other if the opportunity presented itself.
My first step was to join Nextdoor.com and see if this platform held any promise. Maybe if it does, which I think it might, I'll post a sign in the library or in my apartment building encouraging my neighbors to join it, too. Perhaps a non-threatening website might not be as scary to people as some stranger saying "hey, let's get to know each other"
We'll see!