A few tips
Getting them to join:
Tell your friends! Ask them to join and post. This gives the appearance that your forum could be the ‘next big thing.’ No one wants to join an empty forum. Plus, by having your friends posting, you know that they will not spam. By having good quality posts, people will think that your forum is worth joining.
Also, write articles about your topic. Let’s say your forum is about gaming. In this case, you would want reviews about games and consoles. Maybe you would want to compare the PS3 to the Xbox 360 or other systems. By adding new articles, you become a ‘resource.’ People like articles about news, or reviews because they help them decide on doing this or that.
Get a domain and a good forum system. It is very hard to remember a long domain like www.s2.thegreatesthostintheworld.com/digitalgaming/. People want to have a simple domain like digitalgaming.com. They do not want to have to remember a long domain. Also, do not use a bad and ugly forum system. Use an easy to use system. This makes browsing the forums way easier and makes people want to join.
Lastly, skins and Mods are important. No one and I mean NO ONE wants to join a forum with a ugly skin. Better yet, get a customized skin. That way, the skin is only seen on YOUR site. It shows that you care about your site, and are still working on it, and updating it. Mods can also spice up the forum. But beware; make sure that people do not just join the forum to use the mods.
Make sure that they do not abuse the mods:
Lets say you have an arcade system. People will join, just to use the arcade. Now, you think ‘oh, Ill just put a post count limit.’ However, this leads to members spamming. A good way to solve this is to add the mod only to an “Advanced Member” group. This way, you will be able to add only the best members to the group who can sue modifications.
Getting members to return:
Update, update, update!!! Keep adding new articles and posts, to keep the forum alive. Add a newsletter system to keep your members informed of day to day activities. Plus, add new skins and mods to give a fresh look to your forums now and then. Create contests with prizes for the forums (Or if you can afford it, in real life).
Have a kind Community:
I can not stress this enough. I have been to tons of forums in which the moderato0rs are mean, and the admin does not care. The members spam, and you just don’t feel welcome. You must avoid this. If a member does not feel wanted, they will leave. MANUALLY post a welcome message to show that they are welcome in a special way. Also, make sure that at first, they are helped along, and are not flamed. Remember that if the person leaves, they may tell their friends to leave too, or just not even join.