Hi all. I decided to give you couple of tips that work for me and help my community stay active.
The first thing that you want to do when you open a new forum is to show people (guests) that your forum is active, meaning that your boards are full of topics and that your stats are alive and that people can gather info from your community.
1) If you just started a new forum, my advice for you is to start making your own topics, using different accounts. Use a mod for replying to your own posts which is called "Post as Alternative User". But don't post the same topics and replies, replies and topics must be different in word and sentence structure.
Don't log out every time you want to post a reply. Make replies with at least 1-2 sentences, add questions to your posts and make sure to finish topics with "Thanks for Help" , "I appreciate Your Help" , etc. You want to show guests that your forum is extremely friendly and can always help. You also want to show some activity, that's what potential guest (future member) wants to see.
So work on making your own topics and reply to them under different member ID that can be found in Admin -> Members. Your topics should be interesting, don't rush with it, remember, you want to attract attention. Make topics and answers to them in a smart and interesting way. Avoid 1,2 word post.
2) About boards. Don't open 50 boards right away. Do it step by step, take your time. First think about the most important topics that your forum must have. First, make 5 topics and work on filling them out. You want to have at least 15 topics in one board. Think from the side of a guest, when you enter board you want to see many topics in there. You want to have a feeling that there is not enough time to read all of them. That feeling will bring a guest back to the your forum because he/she knows that there is an interesting info.
Imagine if you have 2,3 topics and you already started promotion? A guest came, saw that there are 2 topics in one board and left. You don't want that! Like I said, make at least 5-8 boards and fill each with at least 20 topics. Again, don't rush. Start promoting only after you see that forum looks juicy and always think from guest's side.
3) Promotion. I noticed lots of topics here on forum, saying add link to your forum signatures, promote on youtube, submit articles to digg, etc. The last one, digg - doesn't work for me at all. I've not seen that digg readers registered on my forum. It's just useless in my opinion. Adding link to signatures, youtube videos - works! Do that.
Now, what works the best in my opinion is craiglist guys! If you want to see people registering on your forum day and night then make short ads and post them on craiglist. You want to write a short and positive ad about your forum. You want to explain in one two sentences that on this forum you met many friendly people and that you recommend them to take a look and register if they have questions about this and that. Post ads regularly, I would recommend you to do it on Friday, Saturday, Sunday, that's when most of the people sit in front of monitors and look for something cheap on craiglist.
Ads will least for 7 days only over there. You want to make 2-3 ads in different cities. Remember that there are moderators that check all ads. Some of them can be deleted if a mod will check out your account and will see there the same ads but in different cities. So when you post on craiglist, you don't want to show that you're 100% spammer. If you do, you'll receive a notice from your domain company about stopping doing that. don't over submit craiglist, 2-3 ads once/3-4 days.
Don't forget to add "Welcome Topic" mod to your forum and welcome through PM. You want to welcome every person that registers on your forum. Use different member IDs when replying to his/her topics. Replies must be completely different from each other, again you want to show that forum is active.
One more thing that you want to do is to set up treshhold (Admin -> Features and Options) to some unbelievable numbers, for example 30000. Remove "User for the Past 15 Minutes", you don't want to see there 3000. Second, you want to rename "Users Online" to something that will show that there are really 100 currently members online, you may use what I used - "Currently Users Online."
Also, I would suggest you to remove "Most Online Today:" and "Most Online Ever:" because once you'll start promoting your forum all of the guests that will come to your forum will be added to those numbers and after a while there will be a huge number and it will look suspicious, you don't want that.
This will keep that active mood on forum, it will show that there are always members on forum and plus it will attract guests to register and start posting.
These are my tips SMF members. I hope they will help you.