there is not much you need to do to the forum.
phpbb is indexed by google etc, just fine the way it comes out of the box.
you do not need to add any type of SEO to it.
consider phpbb.com, every single post ever made to that board is indexed in google. most posts get indexed on the same day they are made.
there is no SEO secrets. you simply need to have good content that you continually update and get as many inbound links from other sites with good relevant content and you will get ranked higher.
remember, nobody goes to google looking for YOUR website/bulletin board. They go to google looking for information about something. If your website has the best, most relevant information about what they are looking for, then it will show up in the top of their search returns.
robert