Original question: But how do you make clients consider your advice? If they don't like what you have to say, that's fine. Do you ever have clients who think they are designers? (On Designer News)
At the beginning of a project, I try to get a sense of what the client wants: general goals, websites or apps they like, etc. Then when problems come up, I try to explain them in terms of the client's goals.
For example: "You mentioned that you want the focal point of the page to be the 'buy' button. The copy that you gave me is a little long and would push the buy button offscreen for most users. What do you think about shortening the copy so that it's easier to see the buy button?"
Or: "The navigation that you proposed totally makes sense in light of your product lineup, but I'm worried that it might be confusing to users who aren't already familiar with your products. That shopping site that you like avoids complicated navigation by putting a back link at the top of the product page. What do you think of that approach?"
That way, you can reinforce that you're collaborating and working towards a common goal. It's definitely not a guarantee that the client will agree with you, but it can help.