i have always flushed / refilled mine with ATF, (be sure to check your OM, or the cap- to my knowledge, they are supposed to be filled with ATF) annually. I always replace the filter/reservoir as well.
This is the procedure for non-self leveling suspensions...
you can do this pretty easily with a tube tied into the discharge side. remove as much old fluid as you can with a turkey baster, then remove the two hoses at the bottom. determine which is input, and which is discharge. clamp the discharge side onto a bucket. replace your filter / reservoir and tighten down the input side. next have your buddy turn the car over *after filling the resevoir*, fill again, repeat... DONT let the reservoir run dry, or your system will be full of air, and you'll have to check your levels over and over. once flushed, you can adjust the level with a turkey baster.
It will surprise you how fast the system pumps the stuff through there... you will then understand why i replace my filter annually. A pretty easy DIY, just a bit messy if you dont take care. Another suggestion- stuff newspaper under the reservoir before doing anything- this will catch most spills. Also, put some under the car, and under your bucket.