Soon after I bought my Subie I had a major service done and they found the power steering fluid pretty average so that was flushed out and changed. I have since come across this product:
http://www.magnefine.com/html/filtration.html
It's basically a filter that you fit on the oil return line of the power steering or auto trans, which filters out regular particles by paper filtration, but also has a magnet for metal bits and a relief bypass valve in case it all gets blocked up (failsafe).
Not sure of cost yet but if they are $40 or something, having to change it once a year even if it extends the life of the trans/steering for an equal amount of time per filter it sounds pretty good to me.
Edit: They are $52.00 for a "Kit" (not sure what additional parts it includes aside from the filter, maybe just the wormgear clamps and some extra hose? The filter itself is only $36.00 : http://www.magnafilter.com.au/magna-fine.html
Perhaps we can find them cheaper somewhere or buy them in bulk if there is interest?