Want people to stay? Let them leave...
I wouldn't have a definitive answer, but keeping them motivated and enthusiastic is one essential condition.
And how to do that? No universal answer to that one either.
But consider this:
Companies who seduce customers into contracts, and make it excruciatingly difficult for people to crawl out from under them, have to figure their heads out.
Giving people absolute freedom to walk out on you, immediately, is the only way to find out whether you're adding value to people's lives, or chains with blocks of concrete attached to them.
So do you want someone to stay because they love you, or because you're making it impossible for them to do otherwise?
