Basic tenet of an efficient air cooled data center is to force as much of the cold air coming up from the floor (or down from overhead) to go through servers before returning back to the AC unit. Conversely force as much of the server exhaust air to return back to the AC unit without mixing with the cool air. If you can do this effectively then you can raise the AC setpoints to 72 or maybe even 74 degrees and still have servers held to a safe 77 degree level.
How to do this - In a nutshell - look for any leakage in the raised floor and walls below the raised floor and plug it, use hot/cold aisle with long rows and blanking panels, remove perforated tiles from the hot aisles, Arrange hot/cold rows perpendicular to CRACs.
You may still end up with hotspots due to airflow distribution problems underfloor (pipes, cables etc.). These can prevent you from turning down setpoints to save cooling costs. At this point it is appropriate to employ underfloor air movers such as those found at
http://www.adaptivcool.com/hotspotr.