Overheating is the result of a couple things - a leak, insufficient coolant flow, or insufficient air flow.
If you are loosing coolant, you have a leak. Check these areas:
Water pump (mating to block and shaft seal)
possibly intake manifold gasket depending on engine/coolant routing
The best tool for testing for a leak is a radiator pressure tester. This will pressurize the system, to help you find a leak. Clean everything down first so it will be easier to spot leaks. Look for oil/coolant mixing, and smoke in the exhaust to indicate a blown head gasket. Even with a small leak, the system will not be able to build pressure. The system uses pressure to raise the boiling point of the coolant mixture.
Blockages can happen at any point in the cooling system, but are most likely in the radiator and heater core. The best thing to do here is to flush and reverse flush the system. It may be good to flush the radiator, heater core, and block separately. A trick for determining if there is a block in the radiator is to feel around the radiator for cold spots.
A bad thermostat will not allow coolant through and will cause overheating. You will see similar results with a bad water pump. Symptoms are typically overheating with no heat and a cool radiator.
If there is insufficient flow through the radiator, it will not be able to cool effectively. At speeds under ~35mph, your cooling fan is responsible for most of the cooling. This should kick in around 190*-210*. Past 35 mph, the air flowing past the radiator should keep it cool. Running the AC makes it work harder. Make sure there is nothing in the way that could block airflow through the radiator. Make sure any factory plastic pieces are in place as they help direct air flow. If you have an issue with the fans, check fuses, relays, sensors, and wiring.
There are tuning issues that could cause overheating. Retarted timing and a lean air fuel mixture will cause more heat to be transferred to the engine.
I might be missing a few things, but this should be a good start.