1) Link Exchanges with other web sites.
I am not talking about a link popularity farm or anything like that. I am talking about getting valid links for free from other web sites. If you do this alone and are consistent with it your web site referred traffic can eventually outweigh your SE referred traffic.
2) Write Content For Other Web Sites
No matter what your field, you can probably find web sites that carry current information for your chosen topic. Offer to share some of your content or write a feature article for them in exchange for a link back to your site.
Another great traffic driver. However, you have to give some consideration to where you spend your time posting.
For example, before I became an SEO I was a web developer, and I was also a Macromedia Evangelist. As such, I posted thousands of posts regarding Dreamweaver in their forums and sat back and hoped that it might bring in some business. However, my efforts went largely unrewarded.
The reason? It's simple! Everyone who was reading my posts was also a web developer. They did not need to hire one! For the most part they were my peers - not prospective clients.
However, after making the transition from web developer to SEO and continuing my postings there I gained many new clients. All of a sudden I was posting in a place where my services WERE needed and, in fact, were in great demand.
Continue your postings in your trade groups. However, also find the groups that need your service or product and start post there.
That is where the money is.
4) Award Programs
Want to see your hits soar? Win the Netscape "What's Cool" Site of the Day or one of many other prestigious web site awards. You'll get a huge one time increase in traffic.
But even if you win a smaller award, you'll still get hits. The more you win the more hits you get.
Even if you can't start your own you can usually find someone in your field who is running a successful newsletter. Offer to be a guest editor for them.
If you know your subject matter you can probably crank out a good article in less than an hour.
It's easy peasy - and it's free!