Well, make sure you give them a reason to be active. Try and talk with them on a regular basis and get to know them. Then maybe add some cool stuff to your forum. Contests, video hosting, an mp3 player, flash games, etc. The possibilites are endless. Another good thing to do is to make sure that your forum is not dead, and that you as the admin are taking care of it.
You can't MAKE members active, you can however encourage them by being around each day and taking an interest in what they have posted even if it is not your subject.
You need a good couple of Staff behind you that are great friends with you and want you to suceed, so they are around if you are not. Especially with time zones so that someone is around in each zone. (I know I am feeling that at the moment as my main guy Mod across the pond is on holiday and we are all females)
You need lots of topics to interest all sorts of people, you need determination, dedication and even if you get a slow day still post yourself with your staff as people will return.
Use FP as some people who visit your site will like it and come and post now and then or more....
They have to want to stick around and like you and what you have to offer - a friendly welcoming place where people like to pop into.
You will always get some members who are never very active and some that drop off by the wayside - my main bug is members that are used to VB and just don't have the patience to learn new ways.
Take an interest in each and everyone, people like to be noticed and answered in their posts.
Another thing I've noticed forums implementing is a currency. In other words, when member post, they earn a certain number of 'dollars', and they can be used to 'purchase products' from other members. For example, on my forum, I have an entire subforum dedicated to members opening their own stores.
On my forum we make it so members MUST have 5 posts before using our special features such as the Radio, arcade, casino, and shops. And even if you have JUST 10 active members that is 50 posts daily right there. And people wouldnt JUST post to play the games so most likely you will get more posts. XP
Honestly, theres no real way, because if members dont feel that your forum is interesting enough, then most likely they will leave. So best start out with a small contest, then work up to $5. winning contests and such. Make it to where only members with 100+ posts can only enter.
I've also heard that talking to every member that is active helps alot. Like become friends with everybody. Then most liekyl you will have alot more frineds and alot more active members. I've never tried it, because when somebody told me, I stopped the forum thing.
An interesting thing I do on my forum is grant members 5 of the forums' currency if they bring someone else to the forum and make them an active member. The more members, the more currency.
While this may not work with each forum, it could prove useful to kickstart it.
That is true, but then again, they can be the member that they brought, instead of really bringing an extra member, the member can just register again, and once the money is recieved the posts of that extra members account may jusstart to slow down until it stops. I've learned that kinda stuff the hard way.