I work at a web development agency that manages its many clients' product forums. This is a summary of what I have learned are the most neglected things that can aid a forum like this to grow its userbase. I am not trying to tell anybody how to run their forum just sharing my knowhow.

1-Enable the active users.
You should select the most active users in your forum and make them admins. This improves your forum by making it easier for you to keep it free of unwanted spam and also gives the community a sense of belonging. An enabled user will talk about, share and +1 your community forum more and this will help the forum grow.

2-Allow topic diversity.
Removing off-topic posts angers the poster and such off-topic pages could be indexed in google and create for you some traffic from keywords that you would not get . Keep off-topic posts or move them to an off-topic section, but do not delete them. I have seen that keeping such posts can increase your google traffic by up to 25%.

3-Encourage the loyal.
You do not need to give expensive giftsat all, all you need is to put kind words of thanks to the specific individuals by name in your posts or replies to them. Words such “As usual your post is excellent” or such make a big difference.

4-Every person is a troll sometimes
Sometimes a person has just had a bad day . Sometimes certain terms will bring out the worst reaction in a person. Do not judge by the first time, give a guy a chance and do not delete their membership from the first sign of trolling.