Try to get more people from different forum to help out via pm, etc. Also post the forum here and make some friends to join.
make a contest or attract them to be active by giving them some reward if there are have many post...
It's most probably that a lot of the lurkers are from search engines that have just come to look for something quickly and found what they wanted on your site and then leave as they have their information. If you want more sign ups then get your forums listed on some of them forum lists in your niche. People mostly look for them, because they want to find forums to sign up with.
I would also like to know that, lol. I suggested it and found a few while looking, adding my forums and have now forgot them. It's strange that when you don't have a forum and just browse the web, you keep bumping into them and now I have a forums I can't hardly find any, lol.
I've added a couple more forum categories and the response has been really positive. It also makes the forum look a bit more "full" and active. What do you think? Will also be adding a competitions section too..
The best thing to do when adding some new categories/sub forums is to not go over the top and add to many in one go. The more you add in one goes the long it will take to full them up making them sub forums look rather empty.