First, get rid of all that unused cabling under the floor. This is probably one of the oldest and biggest problems out there for data centers. Unused cabling collected under the floor effectively creates an air dam that won't let air flow. It can also cause a noise field due in large part to the fact that there is generally a significant mixture of power and telecommunications cabling.
Next in line is proper arrangement of equipment into hot and cold aisles. Properly managing airflow will help. Hot aisle containment can also assist in areas where blade servers or other high heat generating equipment is in use. Also, you will want to make sure that you have blanking panels in all cabinets where you do not have equipment. There is some equipment out there that does not have a standard RU configuration, but you can put air pillows, foam or some other blocking material in the smaller spaces.
Virtualization is another boost. If you look at power consumption on a chip, from 0-50% utilization, power consumption is nearly linear. However, from 50-75% there is very little incremental power consumption. Virtualization has gotten to a substantial level of maturity, making it a very viable solution to server consolidation.
Lastly, I would say that properly provisioning your power is huge. If you add up the tags on the back of the power supplies to do this, you are likely working in overkill mode. These numbers are always worst case and depending on the manufacturer. You really only use about 1/3-1/2 of that amount.
Incidentally, you will also see some companies that are moving their data centers to areas where power is the cheapest. This is a bold statement as to power costs!
ABOUT THE AUTHOR: As the Global Network Applications Market at The Siemon Company, Carrie Higbie supports the end-user and electronics communities. She has won the "Communication News" Editor's Choice Award for the last two years.