Along with the aforementioned ways of flushing with a hose, Ive used a couple different additives. Ive used radiator flushing fluid (I never looked at the ingredients to see what was in it), CLR, and citric acid. You can pick whichever you want... I didnt document which worked better, but I know that Ive used citric acid to clean pipes, and machinery systems while I was in the Navy. It worked fantastic, is non-caustic to the touch and is delicious.
Here is my steps for a hardcore radiator flush:
1. With the car cold forward flush the car with your perfered method
2. Reverse flush your car with your perfered method.
3. Blow the hoses out with an airhose.
4. Add water and your additive (CLR, radiator cleaner, citric acid, etc.)
5. Turn on your heater full blast and run your car for about 10-15 minutes.
6. Let the car cool down completely.
7. Repeat steps 1 and 2 again.
8. Fill with appropriate ratio of coolant and water for season/location.
Ive thought about using Coca-Cola as a radiator flush liquid, but am unsure what process I would use. I was thinking of running it for 10-15 minutes to get it circulating and then letting it sit overnight possibly. Ive seen what that stuff can do to rust and am impressed. Im just a little worried what would happen to it when you put it under pressure and heat it up to near boiling since its full of CO2.