Is it simply SEF URL's?
Is it meta tags (via plugin or not?)
Is it the Canonical plugin?
Is it the Google SEO plugin (that actually takes your thread title into the URL)?
Is it one of these or a combo?
And why do you think [your answer] is the best way?
My understanding is that the MyBB URLs that you can turn on make them friendlier (hence the name), not necessarily meaning it gets indexed better. ./user-1.html is just a lot nice than ./member.php?action=profile&uid=1, for example. A description meta tag is good, although a dynamic one is better (Google SEO has this), so that the description is different for each page, rather than a generic one for all pages. I think canonical is only really to tell search engines to use one version of a URL that points to the same page as far as I know. The Google SEO plugin is very good though because can turn off some things (you may not want the names of users in the URL, for example), and it generates a sitemap which is good for SEO.
When I was setting up my board last week I had a hiccup w/ Google SEO. I'd done everything right except for one piece of code. It kept telling me I needed to insert it into my .htaccess. I did. Multiple times.
But I think I needed to place that particular line of code FIRST in the order of things. But I've not gone back to try again yet.
Yes, I believe it does need to be the first thing, I'm quite sure I had that issue, I had it lower down, worked when it was at the top.
Since installing Google SEO about a month ago, my site has moved from Page 9 to Page 2 on a "Beyblade" search query.
I have a feeling content is somewhat important to search engine placement, but I'm not paranoid over seo myself so I don't really know.