Back flushing
The best method I've found for flushing a cooling system (short of having a flush machine, like the ones from BM systems) is:
1) Drain small amount of coolant from vehicle.
2) Add cleaner agent, run vehicle for 10-15 minutes, make sure heater is on.
3) Let car cool off a bit.
4) Locate heater core hoses.
5) Remove the heater core hose that "evacuates water" from the heater core - you might have to start the car to figure out which one it is, but basically, you want to see water coming from the core, not the hose. Be careful - water will be warm.
6) Attach an adapter to the heater core, so that you can pump water through the core first, then have it drain out of the hose that you just removed (it needs to make a full circuit through the cooling system to do this, and it will run "backwards").
7) Remove the thermostat (otherwise, when you pump cold water in, you won't get any water circulating as the t-stat will close due to cold water).
8) Position drain under the dangling heater core hose.
9) Start garden hose with a steady stream of water; don't over-do it, you don't want to damage anything (heater core).
10) Flush the car for a good 15-20 minutes. Turn off the hose and let as much fluid drain out (should be clear water) as possible.
11) Mix-up 50/50 mix of antifreeze and distilled water.
12) Remove the adapters, but leave the heater core hose off; prop up the hose so it's higher than the heater core, if possible.
13) Start adding the coolant mix until you see coolant mix coming out of the heater core.
14) Install T-Stat (you'll loose some coolant) and re-attach heater hose to heater core.
15) Top off coolant.
16) Jack-up the front of the car, so that the radiator opening is higher than the heater core. Block the wheels with blocks.
17) Let car run until it reaches operating temperature, add coolant as necessary. Let it idle and burp for a few minutes, then rev engine to about 2500 - 3000 rpm and hold. Coolant level will drop significantly - add coolant to make up the difference, and install the radiator cap.
18) Drive vehicle for about 10-15 minutes.
19) Let cool completely, check fluid levels, top off as necessary.
20) All done, repeat in 24,000 miles or 2 years, which ever comes first (if using normal antifreeze).