Prepare a list of client commitments and stick to them.  Include returning phone calls within a specified period of time.  Send the commitments to clients with every bill.  Offer discounts if you don't live up to any of your commitments.  Give clients a small discount if they send back a "Report Card" with their payment.  Make it look like the ones kids got in the fifties.  Follow up with them on any grade they give below an "A."