Here's what I did. After my fitness site had about 15 pages or so, I started submitting to a few of the free (and cheap) directories. (I did pay for the $299 Yahoo but not sure that's necessary anymore.) I also used SiteSell's Value Exchange program to find other SBIers who have sites similar to mine and we setup a mutually beneficial link exchange. I didn't swap links with many sites...maybe 3 or 4.
BTW, Value Exchange is free for non SBIers but you get more benefits when you own SBI!. See
http://value-exchange.sitesell.com
I continued to build content throughout this whole time. My first traffic started trickling in from MSN (Live). Then Yahoo picked me up. After my SE traffic started coming in, I started getting a lot of traffic from people recommending me in fitness related forums. Again that comes from building a site that is unique and not the same as everyone else's. Then other fitness sites began to find me (I guess through the SE's) and these people would link to me without even asking for a link in return.
I've noticed many SBIers tend to get MSN (Live) traffic first because that's the easiest SE to get listed in. So once you get that initial traffic that's when you have to make sure that traffic stays. And that has nothing to do with SBI!, that's up to you and your content.
In addition, I submitted to ezinearticles.com. I have one fitness article that has had 20,000 views. Again, Google sees that lots of people are using my content so that's another bonus in their book.
Bulding lots of content is also key because once you get listed in a few directories, get traffic from Yahoo, a few links from related sites Google will begin to take notice. They'll also notice you are the "real deal" because your sitemap will continue to grow over time. So they can see "OK this is a real content rich site on this topic."
So it's definitely a cumulative effect but it has to start with the plentiful, targeted content.
Could you do it without SBI!? Absolutely but the average person needs guidance and it's so nice having all the tools in one place to remind you what you need to do to increase your traffic. Plus the SBI! forums are a Godsend because you can fellowship with people just like you having the same challenges, questions, etc.