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.
In order for them to see your point of view, you first have to understand where they are coming from and what the core issues are. Starting asking them questions that make them think and ultimately guide them to a place where they will begin to see values. Listening is a great sales tool.
If you have case studies with real ROI to prove your point, then that is even better.
Show them a competitor. Usually, whoever they want to be like has got good branding, and that extends to their blog, their branding, their emails, etc. (ideally, you'd be able to find their business cards as well). Then you point out that it's not just (the website, the video, etc.) that they contracted you for, but everything your clients or customers see about you.
I'm assuming the clients fall into the micro business or small business size? Because this is typical, mostly due to the fact they've little training on actually running their own business.
I usually relate to them using something of interest. For example, if I had a client who owned a cooking supplies store, I'd explain how a KitchenAid stand mixer costs twice as much, but lasts a lifetime. When they agree, we talk about the value of having spent more to have a product that looks great on the countertop, requires little maintenance, and ultimately costs less.
You have to talk on their level. It's not a lower level than yours, but a different one.