I work at a web development agency that maintains 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 community. I am not trying to tell anybody how to run their forum ;) just sharing my knowledge.

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

2-Accept topic diversity.
Removing off-topic posts angers the users and such off-topic pages could be indexed in google and create for you some search traffic from keywords that you would nototherwise get at all. 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 gifts , 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-Not all trolls are trolls.
Sometimes a person has just had a bad day . Sometimes certain phrases will bring out the worst in a person. Do not judge by the first time, give a guy a chance and do not revoke their membership from the first sign of trolling.