What does this remind you of?
Every time you visit a new website, enter a new airport, visit a new store, examine a new book... the question you ask first off is, "what's this like?"
At a strange airport, if it's 'like' your airport, you know just what to do. It's easy. If it's totally different, you have to stop, regroup, and start to understand what's involved.
If a book has cheap color separations, the wrong sort of gloss on the cover and the wrong hue to the paper, it just feels cheap and self-published and unlikely to be the real deal. It doesn't matter a bit what's inside, who wrote it, anything. You've already decided because this book reminds you of untrustworthy books you've encountered before.
Visit a website with a brown on brown color scheme, a stock photo of a nautilus, some flashing graphics, a bunch of widgets and a typeface that's not quite right, and you've already decided how you feel. Entirely based on the fact that this site is like those sites, and you didn't like those sites.
Meet someone at a conference who is dressed perfectly, with shined shoes and a great suit (but not trying too hard) and you're inclined to trust and respect him... because he reminds you of someone in a similar situation who was trustworthy.
So why do marketers so often miss this shortcut? Before you make what you're going to make, find something you want people to be reminded of. Feel free to discard this model if you want to make a point (the ipod did not remind you of a Sony CD player), but discard it on purpose. If you're writing a book, for example, your goal (probably) isn't to reinvent what it means to be a book. You're merely trying to reinvent the words and ideas. So when it comes to the jacket and the type, steal relentlessly. Your audience will thank you, because it's one less thing to process.
When in doubt, ask your colleagues, "what does this remind you of?"