The first 1-3 months have always been my favorite part of AdWords consulting. I'd find and exploit huge opportunities (usually the ones I pitched to the client to get the gig) which would sometimes lead to double or even triple digit percentage performance gains. That would secure several months of 'babysitting' the account & not doing that much (I'd be busy working on new accounts) but still invoicing for the same amount.
But eventually, those changes wore off in the hearts and minds of clients, and improved performance became the new status quo - month on month gains were expected. Their attitude was saying "What? Only a 4% conversion increase this month? Is that what I'm paying you for?" or worse "wait, hold on, -8% conversions, is your work counter productive?"
Ugh, the client just doesn't get it. They should be happy with the current level of performance, nonstop month on month % gains just aren't really possible in real life. There's always going to be diminishing returns and breakthroughs involved.
Eventually, I figured out a mindset shift that fixes all this nonsense.
The trick is to direct a client's focus on performance in relation to targets.
Here's what most account managers say to their clients when monthly reporting rolls around: "well, spend is +5% up and conversions are -5% down so CPA is -10% down" usually followed by some external excuse (seasonality is a go-to) or a future promise to make up for the loss. To a client's ears, this basically sounds like "I'm basically doing no work and trying to cover it up. I don't know how to improve your account from here, you should probably fire me and hire a more competent professional".
Instead, you could say "spend and conversions are both on target - spend coming in just over budget at 5% over, and minor, insignificant decrease in conversions" which sounds like "thanks to my hard work and intelligence, I've kept your account performing at the right performance level, everything is fine, you can safely go back to playing golf."
So TL;DR: use targets to frame your clients' performance so that they interpret performance in relation to their goals, not simply the month before.