I'd say find your niche of expertise and build your industry trust level in your ability to show results.
I don't use proposal letters. I talk directly with the client or through MSN, email or other telecommunication device. we discuss what will be done and what won't and then draw up the plans.
Thanks @visio, can you share to us the "what will be done" and "what won't" that you usually tell your clients?
Try cold calling. If you are good enough and know what you talking about and show good examples of how what you do works, you will be shocked at the kind of clients that you will get.
From the work done and the services given to the previous client you get more clients. Same story goes again. But this time it is raised to the number of clients. It goes on and on till you are over loaded with work and cannot deliver quality results.
If you allow that to happen. But there's always a solution for this, hire people to work for you and let them do the overloads. You become the boss