I've always just used those Prestone Flush Kits that tap into the heater hose. On another car I had that was just fulled with crap, I ended up taking the radiator to a shop and having it cleaned, and I put a water hose in the top radiator hose w/o the thermostat in it and turned it on. I ran it until the water coming out of the bottom hose was clear. Worked good for me. You also have to unhook both heater hoses and run water thru one for the same effect in the heater core.
Good luck!