I have been using several different techniques to promote my sites lately. These include forum promotion, free directory submission and article marketing. I've been trying to work out what's most beneficial for SEO.
Of course it's not easy to do this. The effect of link building only kicks in after a while, so if you've used several different techniques over, say, a few months, the subsequent increase in search engine traffic could have been caused by any one or two of those techniques, or by all equally. That said, I think I've worked out the answer from recent link building efforts.
Several months ago I was doing a lot of forum signature promotion as well as free directory submissions. These helped, of course. They are also a quick method to get lots of links happening. I do think the free directories were better than the forums generally, because they are usually more category specific.
The reason: In a forum your signature may not be related to the subject being discussed in the thread. Actually, it's quite likely for it not to be. But you're always putting your link in the right category and sub-category in the free online directories. You're also altering your anchor text and targeting different keywords each time. I've seen traffic coming in related to some of the unusual keywords I've been using in these directory submissions, so these must have been the cause.
Lately, though, I've been focusing on article marketing. And the traffic rose noticeably a few weeks after a burst of submissions. Sure, each link takes a lot more work to get. But all that relevant content that you are associating with it is much more valuable in Google's eyes, it seems.
I think the lesson from this is to keep doing all these things. It's always best to vary your efforts, and not put all your SEO eggs in the one basket. But really focus on the article marketing if you have the time.