Claiming typical savings in the 20-40% range with aggressive strategies able to save over 50%, the DOE has put out a report on how data centers can use the tools from the Save Energy Now program.
Not surprisingly, the key to generating these savings is to reduce the amount of energy used to support and cool the servers. Typical practice shows the majority of energy is used for things other than the IT equipment. Best practices enable up to 85% of total energy used to be used directly by IT devices.