Know who you are trying to engage so you can set up the "right" tool, maybe one that most people are already used to instead of reinventing the wheel or choosing something that everybody has to learn
Less is more:
Set up as few tools as possible to avoid overloading your community
Have few channels so discussions are concentrated in one area and people know where to go to
Rather have more activity in one place than no activity in many places because you want people to come back and to find things again
Gradually introduce new tools / ways of engagement, e.g. by polling people, providing a link to something new and seeing if people are interested in using that different tool
Make it clear where people can go for help.
Not seen much anymore: mailing lists for newbies where they can get acquainted with the community and can learn the ropes of posting in that community.
If something doesn't work for the community, try to find out how to change things so people get more engaged. This might mean changing the tools or changing the format. For one participant in the session, having frequent webinars was very successful for bringing people into the discussion, but still not much engagement between the individual sessions.
Make sure your metrics are right: if 1 out of 1,000 people posts in the community, you might actually be doing quite well.
As community support person / moderator etc., be visible in the community, answer questions frequently, comment so people "see" you and know that their posts are read. Nothing is worst than posting into the void and not knowing who might see it.
You can target specific people and invite them into the discussion, e.g. via email. Idea: Send your post not only to the mailing list but also to others in the field whom you would like to join the discussion. When they reply to all, they are also posting to the mailing list. Problem: Privacy and making it clear to people that their mail can be read by many people.
It's easier to walk away from a forum than from a mailing list because not everyone has your email.
Forums that do not allow for email notifications are difficult to keep track off esp. for newbies who may not know the technology well or do not have time to check online frequently to see if somebody posted something. -> Make it easy for people to stay part of the conversation.
Encourage users through other channels to participate, e.g. mention it in the newsletter, tweet about it etc.
Don't assume that everybody knows where to go. Repeatedly state where discussions take place because there are always new people who do not yet know.
Have FAQs for how to participate in the discussions:
Pro: People learn about accepted behavior
Con: People don't read it anyway, why bother?
Active maintenance: e.g. some communities have conventions, e.g. for when things are resolved; have ratings, follow-ups
Rating system: What are you rating? Do you rate the software, the contribution? How can you be consistent in your rating? Are ratings always good?
-> a number of possibilities to look into as they serve different purposes
Does it have to be a rating system or can it just be an acknowledgment?
How far do you want to go? It doesn't work for everyone.
Create fun badges that people want to earn.
Don't tell everything, e.g. don't give the rules for how to reach the top 2 levels. That adds to the fun and mystery because people also try to figure out how to get there.
Who are you collecting feedback for? The developers or for improving the community?
Debian has a bi-weekly newsletter http://www.debian.org/News/weekly/ in which interesting bits and pieces are pointed out -> point people to stuff that they might want to follow up
Also look at the moderator(s): How to keep them motivated, on board and contributing. Sometimes a "Thank you" from a person who's question was answered can go a long way. That doesn't mean that "Thank yous" should be written all the time and for every response. Some communities actually frown upon saying "Thank you" only as that doesn't contribute to the discussion and just uses space. But in another community it might be fine or even accepted.
Make it personal: You can also write personal messages and see if they give different information, engage more openly -> then find ways to push that public, e.g. by either asking them to do so themselves or by posting a comment of your own