See part 1 here.
In part 1 we talked about finding leaks and airflow problems. But what if that didn't solve the problem? If your fan is working and you can't find any leaks, but revving the engine while idling does reduce the temperature, there are a couple of other possibilities.
One is your water pump. There are plenty of things that can go wrong here, but you can usually tell if it is making noise, meaning there is a problem with the impeller. Check the belt as well. It shouldn't be slipping or squealing; if it is change the belt. Finally, check for contamination in the coolant by looking for crud when you remove the radiator cap. A radiator flush may take care of the problem. Flushing the radiator is a good maintenance practice in general. If the problem still eludes you, check for air trapped in the radiator hoses. With the radiator cap off, squeeze the hoses to try and remove the trapped air. This doesn't always work though, and you may need a mechanic.
Water pump.
If your fan is working and there is no leak, but revving the idling engine doesn't bring down the temperature gauge, the problem is most likely with your thermostat or water pump. You might need to replace the pump if you don't feel noticeable heat when you turn on your heater and crank it up. If the heater is working fine, squeeze the radiator intake hose. It should feel hot, and you should be able to tell if coolant is flowing through it once your engine is fully warmed up. If not, then the coolant is just being cycled through the engine without going through your radiator; this is a sign that you need a new thermostat.
Thermostat.