ok let me start his one.  list below your best forum seo tip.

Yes, I agree. You need lots of content. You also need activity so that there is always fresh content. Search engines rank sites higher if the sites have frequent updates.

Here is a great tip.  Use a specific keyword oriented (or search phrase relevant) title to all your posts.  The thread title is often used in many ways (eg: as link anchor text, in the title tag of the page etc.)  Therefore crafting the thread title to maximize its appeal to search engines is VERY important.

I have found that having too high of a kw density can actually hurt.  I've been told 12% primary KW density is ideal.  I shoot for 10-15%.

I am going to join the content side.  I agree that fresh content coming in consistantly will help the most.  Also, I took the honor of doing random searches in google and waited for a forum to pop up.  Most of the time, unless the person is specifically searching for a forum, they will get a certain thread.  I have also found that almost all of these threads were comprised of:

1) One very long, informative post to start off the thread

2) The original post consisted of header tags , bold tags, italics tags, etc.  This showed the search engine that this content was relevant and rich in information

3) Atleast three responding posts with keywords

Hope that helps....

Good for you.  I often find myself doing things not for the benefit of my users, but for the benefit of search engines.  Which receives the benefit is often mutually exclusive, and it is slowly destroying the net. /endrant

As it is being discussed, I also say CONTENT is the king.

If you have loads of content, then there will be more words in your forum which can expose to Search Engines. And even make the forum links SEO friendly.