Most if not all cars today do not recommend transmission "flushes" anymore simply just drain and refill (I don't necessarily mean you but your mechanic.) This only removes roughly 1/4 - 1/3 of the fluid in the entire system but doing this every 30,000(ish) miles will be enough to keep it in good condition. Some people I know who do it themselves even do a change, drive the car for a day then drain/refill again. Theory here is the newer fluid mixes with the old and changing it again gets more older fluid out. I've always had mine done at 30,000 - 40,000 miles lifetime fluid or not, same with the coolant. at about 200.00 each time your trans will really thank you.
