The number of participants in a particular open source forum, is just a fraction of the total number of developers actively using that product. Think of how many open source projects you use yourself, and compare that to the number of forums you actively participate in. Helping to ignite an open source community may be as simple as asking a few questions.
Sign me up!
If you are a regular user of a community project, sign up to their forum. Introduce yourself and let everyone know what you are doing. Even if that is the only post you ever make, the community will be a better place for it.
There's no such thing as a dumb question.
Ahem... dumb questions annoy everyone. We're all entitled to a few but always make an effort to do some research first. Check the project's documentation, search for related threads in the forum and only then fire off a question.
No one likes to discourage new users, but if your question can be typed into Google and the answer displayed in the first ten results you will be treated with the disdain you deserve -- you are a buffoon. Try and be responsible.
"God helps those that help themselves."
That's a fairly pragmatic reflection from Algernon Sydney (1698); the Bible teaches the opposite.
It applies to all developers with a God complex -- perhaps that should just be "all developers". You would be astounded how much better your responses are going to be if you can genuinely start a forum post with, "I've looked at the docs, and tried a few things but I'm still stumped... any help would be much appreciated."
Developers are motivated by community activity.
No matter how much developers get frustrated by the antics of users in their community forums, frustration is infinitely better than "total fucking silence".
Questions asked means people are using the code base. Questions shape changes, and influence the roadmap. Questions motivate developers to do better.
I know that one!
If you see a question that you know the answer to, help out. A community forum really comes alive if questions get answered quickly -- it gets new users on board, and makes everyone feel just peachy.
If you can do anything to cut down the volume of responses that project developers have to contend with, it helps them focus on the tougher questions that maybe only they can answer. Better yet, they may be able to get on with the development of the project itself rather than using up their time on the forum.
Beat Trolls With Kindness
That's beat as in bludgeon.
A project is never going to be all things to all users. A community forum is not a location for priority support.
People who demand immediate support, criticise without suggesting improvement, and complain incessantly about lack of documentation are buffoons. Nothing makes a community fractious faster than rude users.
Open source developers are motivated by their own needs, in their own time. Putting your code out there and baring all makes you particularly sensitive to abuse. Maybe overly sensitive.
You can really help by responding politely to aggressive users -- there is no point sinking to their level. Funnily enough, a show of polite solidarity from members of the community boosts the morale of everyone -- especially the developers.
A couple of caveats aside... Ask questions. It brings a community to life.