The question was "How do you maintain excellent client service levels when an Account Manager leaves or you acquire a new client?" It's something we've given a lot of thought to over the last year, so I thought I'd share my answer here.
I'd say there's three main things you can do to maintain excellent client service levels in an ad agency.
The first is having processes in place that ensure communication between the client and agency are crystal clear (and that both the account team and client understand those processes). At the root of nearly every client relationship that I've ever seen go bad is poor communication. Conference reports aren't a waste of time. They avoid miscommunication. Creative briefs should be rigorously crafted and should leave no room for misinterpretation.
The second may sound obvious, but it's hiring good people. We always have a handful of account people in mind that we may want to bring on months before we need them. That way, you never have to settle when a new account comes in and we need to hire someone fast.
The third: be willing to walk away. I mentioned above that the #1 reason agencies and clients break up is miscommunication. The #2 reason is that there's just not a right fit. When my agency was 4 people and worked out of the spare bedroom of my apartment (7 years ago) we had a different cost structure than we do today at 30 employees. If you want to be great, you have to add process as you grow in order to maintain the quality of work. It's important to honestly ask "are we a good fit?" and if not, to just walk away. Otherwise, you're putting your client service people in the position of having to justify your agency cost structure again and again. And that's almost guaranteed to lead to a bad relationship.