I have been doing it for years, but in reality I still think it's unconventional. I teach all of my potential clients everything I know. How to build, rank and market their websites. I don't solicit these clients, I just be as helpful as I can.
Because of the nature of my business (Web development) - there is a huge investment in time required to first learn, and then maintain the knowlege needed to be successful. When these students (Potential clients) finally realize that they no longer have the time or ability to keep up, and would rather hire a professional, who do you think they call?

It is the easiest sale to ever make. Most of the time, I know everything about thier website already, and often times know a little about their family, what their hobbies are and what they like and dont like about the web etc. I have earned their trust and they already know i'm the professional.
Now this does take a lot of investment, I probably spend a good 20-30 hours per week posting replies, researching algorithms, staying current in SEO trends, making industry contacts etc, but for me it is well worth it.