Hi M8s! I changed my PS fluide in June 10, at 75K for the first time, and this even if the fluid was looking normal. The easiest way to do it for a none mechanic like me (us):
First, locate the PS reservoir, just behing the coolant and windsheild washer reservoir. Open it, and using a suction device such as a cyringe (big one), I used one with a 6" clear tube a the end to reach all the way in the reservoire. Empty the reservoir, refill it with new ATF-4 PS fluid, specifications (MS-9602). Put the cap back on, then start the engine, turn the stering completely to the left, than completely to the right and return to center, kill the engine and re do the first step again.
Do this until the Qts is empty. The PS fluid capacity is about 1.1 qts, here in Canada we have litres, so I have more than anough. This ensure a complete turn around in the system, and I don't get stuck with left overs.
This is a crude methode, but it does not involve breaking any lines or requiring any special tools and so on, and you keep your $ in your pockets. The process takes about 30 min to complete and the cost is only the PS fluid. Please ensure to properly dispose of your used lubricants.
General rule of thum, change your fluids every 2 Yrs, brakes, PS, and Coolant, the 2 Yrs rule does not apply for the engine oil naturaly.