The flush kits like the ones Prestone makes work fairly well. You can buy a bottle of radiator cleaner solution, and a flush kit for probably 12-15 $. The flush kit is a plastic tee that splices into the heater hose. You fasten a garden hose to the tee, and run water backwards through the system, flushing all sorts of gunk out of the system. You can then add the cleaner solution, drive the vehicle for a few minutes or a few hours (depending on the brand of cleaner) and then flush with the garden hose again. I just did this on my Crown Victoria, and I got a tremendous amount of junk out of the radiator and engine block.
