You have a great relationship with your client (let's call him John). John is the Marketing Manager at Mythco. He knows you understand his business; you consistently deliver effective, creative work; and you always remember to send a message to John on his birthday. In return, your agency is rewarded with a steady flow of profitable work.
A most dangerous time, in this healthy business relationship, will be the day that John leaves Mythco or is moved to another part of the business.
If you are especially fortunate, you may be able to continue to work with John once he is settled at his new company, which will then become a new client for you. But how can you ensure that your relationship with Mythco stays strong once John has left?
Suppliers are often chosen based on strong personal relationships as much as the quality of agency output. Therefore, it is highly likely that your new Mythco contact will bring her own supplier relationships with her, leaving you either out in the cold, or fighting to prove your worth.
It's important never to take your a client like Mythco for granted. If your primary contact leaves, you need to get in there - as soon as your new contact starts - and start forming new relationship bonds.
What you don't want is to have your account go up for a multi-agency pitch (or - worse yet - given to another agency with no opportunity to pitch). A great way to avoid this situation is to treat your beloved client like they are a brand new client, and pitch your agency to them. It doesn't have to be formal, but it does have to be professional, thoughtful, thorough and with a heightened sense of determination to retain their business.
Fortunately, you will already be on the front-foot with "insider knowledge". You will likely know more about aspects of your client's business than they do, and you will be able to support your new contact through their learning phase. This factor becomes an important part of the pitch, as does talking about results, historical work, and your vision and passion for your client's business.
Ultimately, they have to like you. You can give your new contact 101 compelling reasons to continue working with your agency, but if she doesn't click with you personally it's not going to work (or, at least, not for long).