Smashing magazine published this great article about seven principles of clean and optimized css, which covers the following topics:

1. Use Shorthand
2. Axe the Hacks
3. Use whitespace wisely
4. Prune frameworks and resets
5. Future-proof your CSS
6. Document your work
7. Make use of compression

As usual, it has tons of references worthy to check, for instante here from Eric Meyer’s Reset, and his reasoning about why we should reset the css attributes.