For example, clients who insist on using amateurs (sometimes themselves) to create a logo rather than hiring professionals?
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.