Jakob Nielsen has published his annual list of design mistakes. Read the whole thing, of course, but here’s one to pay attention to:
7. Anything That Looks Like an Advertisement
Selective attention is very powerful, and Web users have learned to stop paying attention to any ads that get in the way of their goal-driven navigation. (The main exception being text-only search-engine ads.)Unfortunately, users also ignore legitimate design elements that look like prevalent forms of advertising. After all, when you ignore something, you don’t study it in detail to find out what it is.
Therefore, it is best to avoid any designs that look like advertisements.