Put a plug in the pan and drain and refill at every oil change, or every other oil change. Change the filter at the recommended intervals. Over the life of the trans the effect is the same as a flush, and the price impact is more spread out. Also some have said there is a "shock" to the trans when all fresh fluid is introduced to an older trans that hasn't been kept up, and the fresh detergents in the fresh fluid may loosen deposits all at once, plugging things up.
Shops I've asked don't want to just drain and refill the trans. Last quote I got was $70 just to drain and refill with 4 qts! If you go with the drain and refill, you'll probably have to do it yourself. At least you always know what fluid is going in when you do it yourself.