451 Group has released "Highly Energy-Efficient Data Centers in Practice, a report that examines six green practices used by organizations to reduce data center operating costs and they are detailed in this InfoWorld Article.
In short, the 6 are:
1: Take an integrated and holistic approach to efficiency
2: Be smarter about cooling
3: Generate your own power
4: Save watts with DC power
5: Use your IT equipment more efficiently
6: Go the modular route