If you have a custom built, turbo-charged, super-charged, or other high output engine in your car, you probably need better cooling. You might even consider a new high performance radiator. If you've put in a new radiator and you're worried it's not quite enough, here are a few radiator cooling tips to get a little more efficient out of your radiator cooling system:
Don't use a 50/50 anti-freeze mix.
Water is more efficient at transferring heat than anti-freeze. If you live in a climate where 50/50 is not strictly required (i.e. it never goes below -34 degrees), then you can probably get away with a 70/30 or less (but always leave at least a little anti-freeze in the system for corrosion and build-up protection).
Use bigger hoses.
Fatter hoses let your coolant move more quickly through the system. With less resistance, the water pump doesn't have to work as hard, so you may get a little more coolant throughput or a little less draw of power from the engine (both good things).
Get a new pump.
This is especially true for older cars. Old pumps are just not very efficient by today's standards. Today, you can get high output pumps that move more coolant with less power, leaving your engine cooler and with more power available to spin those wheels.
Never use a reducer.
Reducers are a big myth. Let your thermostat determine when and when not to move coolant. And when you are moving coolant, you want to move as much as you can, especially in high horsepower environments.
Raise the pressure.
Higher pressure means a higher boiling point. A higher boiling point means more efficient cooling. A new radiator cap is usually all that is required for this (but make sure you don't overpressurize - you'll balloon your new radiator cores and wreck the whole system).
Add more surface area.
If none of the other radiator cooling tips can get your engine cooled enough, you probably have to add a bigger new radiator. No matter what a radiator is made from - copper, brass, aluminum, steel, lead - the more surface area, the better. Of course, each type of material has its own advantages and disadvantages and you will have to figure out what suits your requirements best.