A true flush is the way to go, but it can get a little messy. If ya don't even wanna get your hands dirty, ya can try a quick method that does an okay job, but not quite as good as a flush.
Get a gallon of Dexron III tranny fluid. Take off the reservoir cap and pull straight up on the baffle with pliers to pop it out. Take out all the fluid with a turkey baster thing and refill with fresh fluid. Put on the cap and start up to circulate for a short time. Repeat that four or five times. Put the baffle back in and maybe do it all again a week later. It's real quick and easy. Ya won't get out every drop of the old fluid, but you'll come pretty close and your fluid will look a whole lot better.
From automedia.com- "Just as with fluid maintenance for your cooling system and transmission, it's not necessary to change all of the power steering system's fluid to keep it in good condition—as long as it's being done before visible deterioration occurs."
Mark