It's all about the content as mentioned...And the way we can improve content is:
1/ Try to use key words in your forum names, but without them looking/sounding spammy.
2/ If your forum descriptions are showing (Slight edit to viewforum_body.html and viewtopic_body.html) again use key words to describe the forum without looking/sounding spammy.
3/ Have as much activity as you can, if the forum is quiet, post lots of interesting and 'On topic' things yourself.
4/ Don't have loads of forums for the sake of it, empty forums don't help.
Just a few to get you going.
Edit: Almost forgot! Lastly, be sure you have a good host and that your site works speedily! May sound silly, but from my experience this is very important! I had a host who ended up not being capable of running phpBB, the site (Which is new) became almost unusable. I switched hosts, and all of a sudden people are registering and posting. Probably part coincidence, but I think it has definitely played a part!