Is there a reason you want it to run cooler? The engine should run at a certain temp (whatever temp your thermostat is that allows coolant to flow through the engine). When you turn your vehicle on, you want it to heat up to operating temperature as quickly as possible so that it runs as it should. You most likely have a 195 degree thermostat, so when the engine gets to 195 degrees, the thermostat opens and coolant maintains that temperature, additionally, the fan should come on at 195 and cool the coolant. In this manner, your engine should run at a constant 195 degrees once warmed up. If it is overheating past this, than your water pump may need replacing, your fan may be undersized/powered, your radiator could be bad, you could have head gasket issues, your transmission (if automatic) could be overheating and thus overheating the radiator, etc...
Running the engine cooler than it is designed to run at is not good for the motor or performance... Also, when you are on the highway and going fast, the air flow takes over the fans job and the fan sometimes shuts off because it is not needed. This is good, because when the fan is not running, then it is not drawing amps from your alternator, and thus your engine has that much less resistance and more power....
