Over the years, I have thought about this a bit. My current answer to this question is to be mindful of your own emotions when dealing with clients.
If a client criticizes you unfairly or makes you angry, stop writing that letter or email and let things cool off. You have may have to write to the client to correct the record, but do not do so when you are angry. Angry lawyers do dumb things like (a) disclose a client confidence to a third party; (b) threaten to file an ARDC grievance against another lawyer; or (c) file an angry motion with a judge.
When you are angry, you are not thinking correctly. Take time and cool off. Discuss the matter with a colleague or an experienced ethics lawyer. Then it may be necessary to write back to the client and inform that person or corporation of your opinion. Managing your own emotions is the most difficult challenge you will face in dealing with an unhappy client. Remember that the client may be dissatisfied with the result of the matter and that he has emotions as well.