There are at least four key principles of visual design that have an important impact on usability. These four principles — contrast, repetition, alignment and proximity — were originally given the engaging acronym CRAP

Handy guide to A/B testing - the real value however is in the case studies at the bottom. If anything, they prove to me that there are no general rules you can apply to everything (i.e. green buttons). But that you have to test and see what works for you!

Nice book from the people who made basecamp - available free if you don't mind reading on the internets.

It's an interesting discussion... web designers say no, but clients seem to think differently. Should clients get what they want is the question we need to ask...?