1. Use description and keywords meta's in your SMF forum, if this support, or search for some mods/patches for this. Anyway your description need to be bigger than 50 chars and smaller than 250 (maximum). I recommend descriptions to be around 200 chars.
2.Use h1 for your titles. YES is a problem even with smf with this. Try to edit your templates and add h1 on your titles .... but not on all titles ... just in first title from a thread :)
3. One of SEO problems most simple machines forums SMF face is probably duplicate and irrelevant contents. Some advices to skip this problems:
Someone started the same thread on several forums, if you detect that then you can choose what to do with this thread (move to recycle, delete or leave it.). Also if someone write an entire article on your forum just check with copyscape to see if this is already existing in Google index or not. If exists it is your decision if you keep that article or remove the entire thread.
Also from time to time is good to check if your articles are in other forums/blogs and if you found them in other forums/blogs try to talk with the admin from there to remove them or to put a backlink to the original article.
4.Very short threads: If SE doesn't find anything relevant on your thread then is possible to go in “Supplemental results”.
5.Your forum doesn't have a good internal linking structure. Pay attention on every link on your forum and if are links to non relevant pages (login/registration/calendar) add a nofollow attribute to this anchor tags. In this way you will create a tight internal linking.
6.Get rid of session ID's and use some url beautifiers for your forum. I don't know if this exists for every forum but I know it exists for SMF (seo4smf) :)
7.Install a 301 redirect in your .htacess from your non-www to www or use google webmasters tools and set your preferred domain (just for google so is good to add a 301 redirect).
8.A lot of forums have a big problem ... multiple links to the same article for example forum/index.php?topic=12&message=X, forum/index.php?topic=12&prevnext=next for all this links you need to have and the main article without noindex need to be forum/index.php?topic=12. You need to check your forum to see if this is really working in your forum and if not just try to find some patches what is doing that.
9.Improve quality of your forum posts ... Yes I know you cannot control what an user writes but you can add a recycle Bin on your forum and move irrelevant posts there.
10.Try to remove index.php from your forum and redirect index.php to /
11.Try to install a url beautifier because google or other SE doesn't like too many variables in your url.
12.Another problem on forum is the dynamic part of it. Look on your forum and be sure when you access a page what doesn't exits your forum return a 404 and not a 200 message error.