Keen Flo makes a rad flush solution that does an okay job, but the best way to flush the system is to remove the thermostat and flood the cooling system with a garden hose (after you drain the gunk from the system of course!). Take off the top hose and run water until it is clear.
After you change the stuff out, be on the lookout for coolant leaks afterwards. I changed out the (ten-year old) coolant on a Blazer and wound up changing the waterpump out a week later.
