Just for you, snagged from the internet:
You're not the only owner to turn that knob on the bottom of the radiator and have "nothing" of coolant drain out. Here's what I do: (ensure the engine has cooled down compeletly before attempting to drain the coolant, serious burns can occur) SAFETY ALWAYS! Have a bucket or container to collect the coolant. It's against the law to let coolant drain on the roadside. (1)to gain access to the lower radiator hose you must remove the air cleaner housing cover compeletely. Only two bolts and loosen the two hose clamps so you can pull it out. (2) remove the radiator cap (3) Loosen the bottom radiator hose clamp at the point where it connects with the engine hose inlet. It may have a pressure clamp, so just apply pressure to the pressure tips with a pair of pliers and pull it back over the hose. (4) Pull the radiator hose back to remove. Spin it slightly left and right as you pull. Let the coolant drain into the bucket.
Once it has drained compeletly, leave the radiator hose off and draining into the bucket. Place your water hose into the radiator and flush it with clean water for a couple of minutes. Replace the radiator hose, fill the radiator with water and redrain by removing the hose again (don't start the engine). That's it. Reattach the radiator hose and clamp. Reassemble everything else in reverse order. While your at it, you should remove and wash the coolant reservior too. Once you've checked all your connections, bolts, clamps...etc, refill your radiator and reservior with the correct coolant. Make sure you haven't left any tools in the engine compartment. Start your engine and let it run until it reaches operating temperature. Shut it off and after it cools down, check the radiator and reservior coolant level. Add if necessary.
This procedure is a little time consuming, but it gets a nice coolant flush. Good luck!
See? I'm not such a bad guy....