Usually forums are a difficult beast to optimize for Search Engines. Unlike other websites, you as the forum webmaster will face problems like lack of unique and useful content, inability to optimize keywords for certain pages, duplicate content problems and so on.

Difficult onpage optimization. Yes, if you have a website, you can simply do onpage optimization yourself, like bolding keywords, h1 and h2 tags with keywords and so on. But how to do that in a forum? A forum is about letting your users start a topic in a thread, but it's very unlikely they are going to bold keywords for you, they just post. Normal Onpage optimization, a effective tool for Search Engine Optimization (SEO), is sadly out of your hands.

So what's the solution?

Help generate interest in those threads where you know your users will post important keywords by replying or by adding more information. Even if they don't bold it, having a decent number of keywords in the thread will help Search Engine understand what your thread (and hence your page) is all about. You yourself can add keywords when you reply but please make it look natural. You can do the same for links and anchor text.

Content. Unlike websites or even blogs, forums usually don't have unique content. Your users will copy information from other sources and post them in your forum. Even if someone were to post original content, it may be short or incomplete enough to be effective for SEO.

Help me then.

Well, you can write your own articles and post them in your forum. Encourage your users to write articles and have them post in your forum. Have a contest or something. You can even start a controversial topic and this will definitely generate replies and hence content. What you need is user participation and input, without them, your forum doesn't stand a chance and most search engine will just ignore your forum due to the lack of unique content. However, you should concentrate on content which has the important keywords associated to them.

Supplemental Results. One of the biggest problem with forums is that a lot of pages tend to go into supplemental results. Why? Because the threads(pages) have very little information or content. And when it doesn't have much content in it, it looks exactly like another thread(page) with very little information. Your forum is usually dynamic and template driven, which means only the actual content is different, the rest of the forum remains the same. If you strip out all the tags, the menus, headers & footers is the same for all the pages. For example, there is a thread started by A Forumer, and he writes only a few lines: "I have found a cool site! link here! Check it out". And then there is another thread started by B Forumer, and he too writes a few lines: "Nice article! Link here! Good read!". So Googlebot comes and read your pages and due to the lack of content, they believe that these 2 threads are the same (due to your template) and may send one of them to Supplemental Results. I am pretty sure you have a lot of such posts in your forum, right?

What can you do?

Simple, if someone posts such a thread in your forum, just go into the link and extract some summary and paste it in the thread. For the above article example, goto the article itself and copy several lines into that thread. This way, there is additional content for Googlebot to index and gives you a less likelihood of going supplemental. Encourage more users to reply to the thread can help as well. You should also check all outgoing links your uses have posted and see whether these links lead to blacklisted sites. If they do, delete them off immediately.

You can also look for forum modification scripts which helps you change the meta descriptions of your threads so that it would look different from each other to Search Engines and reducing the risk of your pages going into supplemental.

There are some modifications scripts out there which can help your improve your forums' Search Engine friendliness by creating Search Engine friendly static URLs, or even keyword rich URLs through mod rewrite. In additon, see whether there are scripts which can help you create sitemaps for submission to Google to help them index your forum more efficiently.

Another effective way to improve your forum SE-friendliness is through archiving. Archiving means stripping everything out except the main content and important links. This way, Search Engine can easily and quickly crawl and index your forum without the rest of the rubbish cluttering it.

There are a lot of other ways to optimize your forum, too many for this article anyway. But you should follow some of the above tips (especially those tips which doesn't require additional coding skills) as they are pretty easy and they require only a minimal effort. Basically, they all have to do with adding content and keywords into your forum posts but still making it look natural.