Josh and I were talking about SEO ideas last week and he mentioned a novel one. Search engines are trying to find the best content for their users, so a nice approach to getting good search engine rankings is to create a useful site with great content that people want to visit. Of course that doesn't happen overnight, and having the correct title, meta tags and keyword density don't hurt, either. But the fact remains, if you create something that people want to visit, you will get inbound links that will, in turn, increase your search engine visibility.
It kind of reminded me of an exchange study in high school I knew. When he lived in Spain, he had a tough test in which he must have spent at least 8 hours using the tip of a compass to write the answers into the side of his pen. He showed us the pen, and you had to be about 1 inch from the pen to read anything! Obviously, if he just spent those 8 hours studying, he probably would have got the same grade and would actually be smarter.
Same thing, to a degree, with SEO. Learn to be an expert in your field and spread your knowledge across the web with blogs, newsletters, guest blogging, etc. In the age of social media and user interaction, if you truly have something to say, people will find you.
The point is: paid search marketing and organic SEO measures definitely are part of the pie, but they have to be backed by substance to turn into conversions. Give your potential clients a useful website and they will use it and tell other people about it.