I own a MyBB forum. I have installed SEO plugin to make it user friendly and to have a sitemap.
These are the features:

  • Google SEO 404:
    • Error pages return HTTP 404 Not Found (and others) instead of 200 OK
    • Custom 404 error pages
    • Google's 404 widget

  • Google SEO Meta:
    • description meta tags for forums / threads / etc.
    • specify canonical pages
    • nofollow links

  • Google SEO Redirect:
    • redirect old URLs to...

Where to start to optimize a forum?