Since it launched in May, you’ve probably already seen this, but I found it just the other day and I think it’s a very nice & useful website. Created by Daniel Higginbotham, as the tagline summarises, it’s a guide that explains how to achieve a cleaner design, and it does so in a very clear and straightforward way, using simple language and a lot of visual examples. A good starting place. 

Enjoy: Clean Up Your Mess — A Guide to Visual Design for Everyone.