For example, clients who insist on using amateurs (sometimes themselves) to create a logo rather than hiring professionals?
We share a similar challenge -- visual for you, verbal for me.
In my case, this often means convincing folks that cutting corners on their branded domain name or written content carries hidden costs. They often see spending more than the bare minimum on their online identity as waste.
It's easy to be oblivious to the costs of bad branding or bad design. After all, they have a website already; so they've already arrived at the finish line, right? People think they're saving money simply because they're not spending money. Meanwhile, they're earning LESS.
It's also tricky to point out blemishes in someone else's creative efforts. Egos are fragile. Or they're too robust!
So what I typically do -- and what I recommend you try -- is this: Gather some compare-and-contrast examples. Ask your prospective clients which of 2 websites or logos they'd rather do business with or which they've already done business with. One will look professional; the other will resemble their site / logo.
After a series of these compare-and-contrast examples, the person will usually grasp what's at stake without any didactic explanation from you. People want to trust their own judgment; therefore showing them is better than telling them.
Frankly, I think you've got it easier than I do! Visual differences are striking, and people are accustomed to the idea that professional graphic design ought to be paid for.
You will only be able to convince a small fraction of people. We human beings are stubborn, ignorant, and lazy. All of us.