I do several things, some already mentioned.
I remove session ids. I add a google sitemap mod that automatically creates a sitemap for the forum. I add a mod that also creates a regular sitemap for the main pages of the forums, and another for the actual forum messages.
I also have some of my own modifcations that set up title, description and keyword tags thoroughout the forums.
I would recommend not using any kind of url rewrite mod for phpbb. It is unnecessary to get your messages indexed, and actually creates more duplicate urls.
I think those are the basics I do.. I do a few other things as primers, but has more to do with the way I use things than any kind of general set up.
Personally, I started with the ABLE2KNow SEO mod, but eventually just morphed it into my own set of modifcations. Like I said though, if you use it, I would not do the url rewrite part of it.