Answer: Give them nice phrases and be open-minded.
Explanation: Here are some examples of what I meant.
“I see what you’re saying. However, given all of the time I have spent in the field and knowing what I do about it, I must say, personally, that I would let the design stand.”
“I only want what’s best for your project. And from all of the satisfied clients I have worked with in the past, I have a very good feel for the market. So trust me when I say that this is your best way forward.”
“Given my extensive background, I firmly believe that this design perfectly satisfies all of your needs.”
“In my professional assessment, which is in part why you hired me, any further alterations would be detrimental to the effectiveness of the design.”
Other than that, Keep your skills and experience at the forefront of their mind. Show them your single-minded passion for the project. Foster in the client a trust in your abilities, so that they become comfortable deferring to you.
I'm sorry if this didn't help.