The purpose of the cooling system is to bring your vehicle up to operating temperature as quick as possible, and also to maintain operating temperature throughout all driving conditions. This is done by the thermostat controlling the amount of coolant flow from your engine to your radiator.

What should be checked?

Condition and strength of coolant/antifreeze.
Radiator and hoses.
Thermostat.
Coolant fan.
Water pump.

A common problem is a thermostat that opens too early, this will not allow your vehicle to properly heat up and contribute to poor fuel mileage.