+1 on what Jim said, maintenance is the key. My dad is the worlds worst, he will not change the tranny fluid, as long as it works, he doesn't mess with it.
If you drain and fill at every oil change (3000 miles), in 200,000 miles thats 67 oil changes, at about 3 bucks per qt for dex III and 2 1/2 qts each time, thats $502.50 and you can easily cut that down to every other oil change if you prefer, so that isn't terribly expensive.
I started doing it every 3k and after 3 or 4 times the fluid coming out started looking as good as the new stuff.
