The way I do it is undo the resevoir to pump line at the pump, and low pressure return from the rack. Let it all drain out, turn the wheel from side to side, this way you get 90% of the old fluid out, then you tighten it up and refill resevoir. The only old oil left is the high pressure side from pump to rack. You will need four new crush rings though (the larger diameter ones), and it's a good idea to replace the resevoir if the car over 200k, as the resevoir has an internal filter. Not mandatory, but for $60 for a new one it's cheaper than replacing a rack or pump.
S52|ZF|TRM Coilovers 670/895|Staggered 18" 5's