Antifreeze Flush

There's a technique I use called back flushing. You can get a kit from the auto parts store for about $5 that will do this. There is a fitting that you add to one of your heater hoses that connects to a garden hose. You then turn the water on, and the water circulates through the engine first, and gushes out of the radiator cap opening.
Once the water runs clear, you can drain it again and fill with coolant.