David Ogilvy again:

Always design your layout for the publication in which it will appear, and never approve it until you have seen how it looks when pasted into that publication…. A layout must relate to the graphic climate of the newspaper or magazine which is to carry it.

Yellow Pages too. We try to always have a copy of last years Yellow Pages, the section in which the ad we are designing will appear. If everyone else is using blues and greens, yellows and reds will make our ad much more noticed.
This rule applies just as well to the Internet. What do competitor websites look like? How are they structured?
Then make sure your website looks different but not too different.
Legal website are particularly bad in this regards. They are virtually indistinguishable, one from another. And certainly no one will remember one over another.