10 Laws of Simplicity
1. Reduce: the simplest way to achieve simplicity is through thoughtful reduction.
2. Organize: Organization makes a system of many appear fewer.
3. Time: Savings in time feel like simplicity.
4. Learn: Knowledge makes everything simpler.
5. Differences: Simplicity and complexity need each other.
6. Context: What lies in the periphery of simplicity is definitely not peripheral.
7. Emotion: More emotions are better than less.
8. Trust: In simplicity we trust.
9. Failure: Some things can never be made simpler.
10. The one: Simplicity is about subtracting the obvious and adding the meaningful.
[From the book The laws of simplicity by John Maeda]
http://www.amazon.com/Laws-Simplicity-Design-Technology-Busi...
Also, from a usability perspective, there are a number of tools people use. Lots of them boil down to "figure out why people can't use this or think it's ugly". But i'm no expert.
Also, it helps to have a sense of aesthetics. That usually comes when you admit something is ugly, and grows from there.
There are a ridiculous number of design books out there. You could probably finish "Laws of Simplicity" in an hour. http://www.amazon.com/Laws-Simplicity-Design-Technology-Busi...
Note that I'm no designer.
Drive: The Surprising Truth About What Motivates Us -- Dan Pink (http://www.amazon.com/Drive-Surprising-Truth-About-Motivates...)
Linchpin: Are You Indispensable? -- Seth Godin (http://www.amazon.com/Linchpin-Are-Indispensable-Seth-Godin/...)
The Laws of Simplicity -- John Maeda (http://www.amazon.com/Laws-Simplicity-Design-Technology-Busi...)
Zen and the Art of Motorcycle Maintenance -- Robert M. Persig (http://www.amazon.com/Zen-Art-Motorcycle-Maintenance-Inquiry...)
Invisible Man -- Ralph Ellison (http://www.amazon.com/Invisible-Man-Ralph-Ellison/dp/0679732...)
How to Win Friends and Influence People -- Dale Carnegie (http://www.amazon.com/How-Win-Friends-Influence-People/dp/06...)
The Kindle app has really got me buying a lot of books that I now need to finish...