"Work smarter not harder" is lame, obvious advice that you should always keep to yourself. Obviously this is what should be done, right? Saying it out loud doesn't serve any purpose other than to mark yourself as the kind of person that nobody wants to hang out with.
Again, though, it's good advice. Folksy knowledge. Common sense.
Here's a paper I found recently that explains why:
Repenning, Nelson P., and John D. Sterman. "Nobody Ever Gets Credit for Fixing Problems That Never Happened: Creating and Sustaining Process Improvement." California Management Review, vol. 43, no. 4, July 2001, pp. 64–88, doi:10.2307/41166101. (pdf) (notes)
Common sense is easy to dismiss when it seems folksy—the sort of thing that doesn't necessarily apply to a Man of Sophistication. So what I like about the paper is that it takes a system dynamics approach to explaining why working smarter eats working harder's lunch. System dynamics (Wikipedia) is essentially a way to understand how feedback loops affect the behavior of a system, which produces weird (non-linear) results.
The short-short version is: working harder is like trying to cut more wood with an ax by spending more time chopping without investing enough time in sharpening the ax. It's like eating the seed corn or spending the principal. In the short term, you might get ahead of the problem, but at the cost of reducing your capability to solve the problem so that eventually you fall behind. And then what? Keep chopping or start sharpening? The former will lead you into the capability trap where you get too far behind to ever recover; the latter will cause short term slowdown when you take away time from catching up, but long term improvements because you can move faster.
Reading this paper will give you some substance to push back on the natural drive to solve a capacity problem by simply spending more time on it.
...Now that I think about it, working smarter might seem like common sense intellectually, but viscerally working harder feels like common sense in the moment—some sort of an I Am The Master Of My Destiny feeling as you wrestle with something that didn't necessarily need to be wrestled with. That's covered in the paper also: the fundamental attribution error, where a manager assumes that the problem is a lazy or incapable staff that just needs the right amount of beating to get motivated to solve the problem. It's not an easy feeling to resist, but if you remember there are benefits to sharpening the ax, you might make the right long term choice to do so.