Take special notice of members that make their first contribution.
If you're reading a post from a member with a '1' post count, your response is a big deal.
Your response needs to be quick. If a member had to wait a few days for a response, they're gone.
Your response has to be genuine. You have to response to what they say. You need to acknowledge the quality of the question and provoke further debate.
You might also want your volunteers/regular members to participate. Responses from 5 people including 2 long-term members is better than a quick response from the community manager.
If you're finding that many of those whom make a first contribution never make a second, you might want to work harder at crafting a better response to that first contribution.