First, you definitely want to drain both the radiator (drain plug at bottom of radiator), and the engine block (drain plug-19 mm-on side of block forward of block heater...reach in with long extension bar. This will completely drain coolant (except for little in the heater core).

The great tip was this: To refill the coolant (after plugs are re-installed!), first disconnect the end of the large radiator/coolant hose at the top of the radiator, and fill as much 50/50 mix as it will take by pouring into the hose. Note that this allows you to fill engine block "behind" thermostat. Then reconnect the hose, and then fill as much as you can into the radiator by pouring into the expansion tank...this fills in "front" of the thermostat.

By doing this, you get coolant into the system on both sides of the thermostat, so you don't have the big air pocket, and the problem with running the car to over heating in order to get the thermostat to "release" and allow coolant to circulate.

I got this tip from an indy mercedes repair shop I happened upon while on the road today...I stopped to buy coolant, and we got to talking, etc....

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)