Monitoring,Benchmarking and Data Center Metrics - This is probably the most important part of any energy saving strategy. You cannot fix what you don't know about or understand.
Here's a good real world example: Company A decided to monitor its power usage and see how it was trending. They found several spikes and anomalies one of which they determined was the CRAC equipment fighting amongst themselves -  one dehumidifing while another was humidifing while another was reheating. A simple networking strategy for their units saved them hundreds per month.
 
 Variable Speed Drives - The idea here is to vary the air flow on chilled water based  cooling equipment based on the actual need for cooling. Also Electronic Contolled fans
on the output side can do similar functions on equipment that uses freon or glycol.
 
 Air or Water Side Economizers  -this approach is more popular in colder climates
but some Southern based data centers have used this along with some other technologies
to tack on extra savings,
 
 Ultrasonic Humidification - this one doesn't seem like much but ultrasonic humidification
uses as much as 70% less energy than steam based units and are virtually maintenance free.
Plus this is one of those hot button technologies that the feds love and is often eligible for rebates.
 
 Virtualization/Consolidation - Yes I know this seems obvious but what we see sometimes are data centers virtualizing and getting smaller...in the same big inefficient room! Take a look at the physical size of your room and see if a smaller room can be created or if not, look at doing a cold aisle containment so that you are cooling the places where the equipment is instead of where it is not .
 
 DC Power instead of AC - This can be kind of tricky but bringing in a good EE can get you where you need to be. Telcoms have been doing this for years and the savings here can be multifold. Reduce the cost of individual servers by going to central power supplies. DC takes a conversion out of the process (DC back to AC) thus it's more efficient.