Are you a change agent?
In Prototype the Experience, Ian White makes a few points that ring true with my experience selling services to my clients:

"The context within which a product exists strongly influences the user experience and provides meaning. Illustrating this context by constructing a scenario around the artifact allows a business audience to understand a solution, rather than see a component which any design firm could provide."

"You owe it to your clients to help them think differently about design. Make them the champion of new products, services and opportunities. You can gloat in their success and be regarded as a powerful change agent."

My take:
Many designers and usability professionals ARE change agents -- but they may not see themselves that way. Change agents have to provide a vision that resonates with their clients. Clients really don't just want to know what you DO, they want to envision what you'll facilitate for their customers, their businesses and their own careers.

Provide inspiration from the start and they'll understand the solutions you provide, otherwise they'll just see the deliverables and the invoice.

"Those who have changed the universe have never done it by changing officials, but always by inspiring the people."
- Napoleon Bonaparte