There are many ways to optimize a phpBB forum. Probably your first step would be to rewrite the URL's to a cleaner and more SEO friendly way. There are also plenty of SEO mods for phpBB that can help you with page titles, descriptions, keywords, etc.
One way. Create good content. Tricks, otherwise known as SEO don't work anywhere near as well as having great content. Focus on that and you'll be indexed well.
RectangleMan is right about content... Focus of building content, fresh and unique quality content... that will spark long discussions, then you will see Google and other search engine bots constantly on your forum digging around All the other "tricks" won't work unless you have content...
URL rewriting is first and a must. Beyond that, content content content. It doesn't matter what else you have in place, keywords, meta tags etc, they're all useless without content.