There a lot of different things to do and not to do when managing a forum.
I have been a member of a countless number of forums as either a member, mod, staff and so on since the 90's and have seen lots of good and bad things being done by the staff/admins.
As far as the do's and don'ts of managing a forum, here are some of the key ones from my experience.
Do's :
Try to be active on the forum and interact with the regular members. This will help in many ways, including knowing what is going on around the boards, giving the regular members a sense that you are one of them, you can react to issues faster and more consistently and there are several other reasons as well for a forum manager to be active. I have either been a member or worked on many different forum sites where there was rarely any staff/mods/admin interaction. Many just do the set it and forget it way of managing. That's one good reason I like Webmaster Sun, there is always staff/admin members around regularly and interacting with regular members.
Another key do in managing a forum is to treat all it's members equally. This will help gain respect and trust from the regular members, which is key to a forums success. Many forums I have been a part of over the years have had staff/mods/admins that treated different members in different ways. They would allow some things from some members/friends of theirs and the same things aren't allowed by other members. This will get your regular members to lose respect and their trust.
One more do that I think is very important to the success of a forum site is to listen to your members as far as changes to the forum. If you're managing a forum, you always want to be able to grow and improve, so if there is a good suggestion that will make the forum better AND it's doable then that's something to consider. Many forums just stay the same and don't grow and improve over time and that can lead to the downfall of the forum.
Another do is to be consistent and run the forum by it's rules so people will know exactly what they can and can't do. This is especially important if there are disruptive members who like to troll or drive away members in some way. If someone is hurting the forum, the worst thing that a forum manager can do is to let it continue. Give warnings and ban if they won't stop hurting the community. When giving warnings, don't just give them one without an explanation as to why they are getting the warning. Explain to that member what they are doing wrong and how it's hurting the forum. Many people don't realize just how much of a PITA they are being and after a manager/staff talks to them they often tone down their harmful ways.
Don'ts :
The first don't on my list was mentioned above. Don't show special treatment for certain members and always try to be consistent with all members, no matter what their role is in the forum. Being consistent will gain the respect and trust of the members and allow for growth and bettering of the forum.
Another don't would be to not act like you are some kind of God, which many do. Too often someone who's managing a forum will act like they are a God or better than everyone else. Doing this will lose members respect and will also cause the loss of a lot of active members. Ego trips are definitely bad for a forum site's future.
Perhaps one of the biggest do's is to promote the forum in as many ways that are good for increasing the amount of good, quality, active members. People can't join a forum and contribute to it's growth and success if they don't know the forum even exists. There are many ways to do this and some depend on what niche the forum is in. In general though, some ways would be to market and promote the forum through social media websites, some PPC advertising, interact and even comment or guest post on relevant blogs/websites to help bring awareness of the forum before more people.
Also, it's always been a good way to keep members active and spreading the word about a forum by having regular contests or giveaway's of some sort. I've been a member of many forums that have done this and it always increases the amount of time members will be on the forum and checking back in. It can also drive new member sign-ups if some advertising or promoting is done through social media, PPC advertising and so on.
There are many different do's and don't that are required in managing a forum site, and many of these can differ from one forum to another. I haven't been a member here long but from what I have seen and experienced, the management/staff/mods seem to be doing very well. Even though this might not be the biggest forum on the web, it has a lot of potential for growth because things are done properly here.
I think and the word is spread more and more about our community here, it will keep growing, getting better and increasing it's membership. I am also going to be someone who helps spread the good word about Webmaster Sun.