I find Peter Block's Flawless Consulting a great way to think about how a person can work as a consultant. He talks about three possible roles for a consultant: the expert, the pair-of-hands, and the collaborator. Each is a valid role, but you and the client need to be clear on which role is relevant to the engagement.

My own experience hints that I often get the job because I'm seen as an expert in some area, but that in many projects I end up as a pair of hands: the client believes he knows what he wants done and how; my charge is to get it done.

(This isn't always my choice, mind you, but it's one way the world works.)

Collaborative engagements, I think, appear after you've developed a network of past partners and clients.

Mark, I'd encourage you to imagine a potential client asking, "What can you do for me that has value for me, and how do I know you'll do it?"