I was talking to a photographer today who appeared to have been working at random jobs in an effort to make a living. Her specialty appeared to be in photography and of course that usually means trying to find clients or your next job all the time. I then mentioned about things like selling stock photos and funny enough she never even thought about it. After mentioning how some photographers make a very good living that way she immediately wanted to investigate into it.
I suppose in many ways that is an over sight for everyone who is self-employed and runs a service based business. If you do offer a great service why not think about the idea of turning your service into a product that you can sell as well? For example, a painter can wait for someone to hire them to paint a project or they can create their own paintings in the meantime as well and sell their work. A good alternative as opposed to only waiting around for people to call you I think.