One important thing is to submit your site via Google Webmaster tools
The phpBB defaults are usually fine for site indexing.
All of my boards are regularly indexed - you just may need to wait a while.
I have default installs with no SEO modifications.
There are a couple of sitemap MODS in the development forum, but I think they are [BETA] so still in testing.
Personally - I've never needed this functionality - I just include my main forum index pages in the sitemap for my main sites, and google quite happily spiders away without any help.
Google spiders the forums regularly, and the links are all good topic links, not nonsense links to search results etc.. Maybe I'm just lucky
Because my sites are relatively small, I just have a urllist.txt in the root of the site, listing urls I'd like google to look at - I never bothered with xml sitemaps yet. So long as that urllist.txt is submitted via Google Webmaster tools it seems to work fine for me.
As Steve says, check the BOTS can browse your forums - you should be fine.
Apart from the meta description I don't think you need to do a lot.
I set up a new forum 10 days ago. After 3 days a search for one word + forum and it was 14th on the list. On 7 days it was 8th. As of now it's 4th. I've done nothing except the meta description, put it in my sig on a few forums I post on and allow the bots into one announcement forum.