Chillers, fans pointed at the surface of the water in the display, fans pointed at the surface of the water in the sump, using cool running (external) pumps and powerheads, using cool running lights (like T5's instead of MH or NO), not putting a top on the tank, extra fans in the canopy or fans pointed at the light fixture, having the light fixture lifted further off the tank, keeping your house cool, etc.
Personally, I am using a clip on fan pointed at the surface of the water, PC lighting lifted off the tank, and no top.