Fans won't do anything directly about temperature. Moving air seems 'cooler' because humans sweat and the evaporation of sweat (which is enhanced by moving air) is what actually has a cooling effect. Plants, on the other hand, don't really sweat - they do loose water to the air, but not so much for the purpose of keeping cool. A fan combined with a humidifier of some sort would do the trick - as humidity goes up, temperature goes down. A cheap way to get that effect is to just get an aquarium air pump, some hose and an airstone. You hook the airstone to the pump and put it in a deep container of water. The bubbling increases the humidity and cools your tank a little. You could also seal your lights away from the terrarium and increase the air circulation to the lights - this would help blow away air warmed by the lights and reduce the amount of heat going into the terrarium.
I've been toying with the idea of rigging a fan and air duct to this old cat door I have, so I could draw in cold air from outside to cool my plants at night. I think I got the idea from a setup by somebody from here on TF, with an inlet from their window to their terrarium. I'm planning to move soon, so I haven't done it yet, but when I build my big highland terrarium, I'm either going to use this method or a refrigeration unit to get consistently cool air.
~Joe