When evaluating which software, research the "mods" (modifications) available first as these will form an esential part of the site as it grows and you don't want to be stuck with crap you can't change. vBulletin wins IMO on this front, but phpBB (free) has plenty of mods many of which are built-in to the new v3. If you have the money, go with vB. Most "serious" forums (this included) use it.
Software aside, there are a few things I'd submit as ideas:
1) Start with the absolute minimum forums possible. While it may make sense (and take time) to seperate topics out later on, having less forums makes your site appear busier early on as all posts are in one place. A busy forum attracts posts. Well, assuming you have traffic
2) If the idea behind the forum is to generate extra pages and build an extensive site, an SEO mod is essential. vBulletin has the best of these. phpBB has one but it's more limited in URL generation.
3) If your forum gets popular, you will get spammers unless you take steps to discourage them. I would disable signatures until a user has made 25 (or more) posts, use groups to handle this. Moderate new users - spammers are often obvious, and definately activate the image recognition thingummy in the registration process.
I even moderate posts on my vBulletin forum. On my phpBB forum I just closed the damn thing and use it now for my own announcements only. Spammers don't have friends to distract them unfortunately so be prepared
Cheers
Ian