Sit down and call, write or email 10 clients you have had in the last 6 months and ask them specifically what it was about you, your business or your offer that made them hire you. If you start to see a common theme, then you know you are doing it right.
If you get 10 different reasons, ask each of them to give you another reason (if any). If these 10 or 15 people are any barometer to your overall marketplace, you can use these answers to fine-tune your sales call process as well as your marketing.