You've probably heard this before, and I realize it may not work for many sites (especially small biz) but I have found GOOGLE is the answer.
I'm not just going to advocate for Google but I can tell you it has worked for me. So the old school rule applies -- content is everything. This also may be because my web site contains over 10,000 pages.
For my small businesess (not content web sites) I have tried different approaches and unfourtunately have not met much success. I tried link exchanges, even channeling some of the traffic from my larger sites into my smaller ones.. but it wasn't very effective. Posting in user groups has met limited success but it just takes so much time... Non-internet marketing has has some limited success but again its expensive and you really have to network.