Seth Godin, marketer, author, and blogger, has a great post about basic web usability.
"If you approve or create online forms or deal with consumer interactions, I hope you'll think about the following:
1. If you have more than seven items in a pull down list, you have failed."
Read about it over on Seth's blog - The magic rule of seven (and the banality of alphabetical order)."
I don't know if Seth is using his intuition, but let's say he is, about web usability or user interfaces to support his hypothesis. It has been my experience as a web developer, web optimizer/tester that intuition in most cases is usually wrong. A better approach is to take a data-driven one and use web analytics.
To fix usability problems and conversion rates - the best game in town is Google Analytics. You can do all of the a/b and multivariant testing you want. You can test any element on a page including the whole page. FREE.
Here are some more ideas about website optimization.