Say sorry. If you're in the wrong (you've overcharged them), apologise personally and sincerely. If you don't feel responsible (their evershifting priorities made delivering on time impossible), apologise for the situation. Now is the time to save your sinking client relationship, not to save face.
Ask meaningful questions. They'll help you clarify what went wrong and show you're serious about finding a solution. Summarise their complaint back to them.
Be honest. Think carefully about what you can and can't deliver. A realistic solution is better than a broken promise.
Empathise. Identify with their complaint on an emotional level as well ('I know how frustrating this must be for you'). Once they see you as a fellow human, they'll be more willing to cooperate.
Don't pass them on. They may not have been your problem originally, but they are now. Tell them exactly what steps you will take personally to resolve their issue and by when.
Join forces. Instead of contradicting the complainer with defensive language ('No, but'), be positive. Start each of your responses with 'yes, and ...' and watch cooperation conquer.
Stay calm. Count to five before speaking, draft and redraft and, before you hit send, breathe. One angry person is quite enough.
Delve deeper. Don't be tempted by quick fixes (discount their next order and move on). Most complaints are products of a more serious problem (eg, an inefficient delivery pipeline). Keep digging until you find the root of the complaint, then tackle it.
Be thankful. Complaints are opportunities to improve, innovate and win over clients for life. They're just in disguise.
- The Mind Gym: Relationships is published by Little, Brown at £12.99 - www.themindgym.com/books