Hi! I'm relavitely a newbie at all of this (if not I probably wouldn't have started two groups, one in Yahoo and one in MSN.) I'll just give you my two cents about what's working for me. I don't have a lot of members on either one, but nice things are happening, people chat, people email, share, I get messages. So the stuff is moving, which is what I want. I wouldn't want to have hundreds of names and no visitors nor activity.
I do these simple things. 1. Every week I post a special feature (an article for example) that gets everybody talking, and giving their opinion.
2. I offer freebies (like ebooks)
Last week a lady that heads another group for strokees (recovering victims of stroke), joined my grup at Yahoo and passed our address and ebook to her members! If you offer stuff people need or want, they'll come.
3. Every now and then I send each member our "Occasional" Newsletter, which, since it doesn't come out all the time and I try to pack with useful and great stuff, to them, is something special. I have had people sign into my mailing list through Veggies Unite just to get the Occasional Newsletter (knowing it will be a special thing that comes along only several times a year.)
Brainstorm, come up with something that other forums of your type are not doing. For example, for me as an inexperienced user, it was quite devastating to be invaded by malaware and ad-ware, why not put out a e-newsletter every now and then warning your members about "latest" thread, how to recognize de symptoms and get rid of the malady. Something that will make you unique and will make people flock to your forum, saying, "they've got the stuff I need." Or tell their friends, "Go to such and such forum and you'll find the answer to that." My sister got me to join DaniWeb. Never forget the power of human promos! Hope this helps.
Take care!