i do mine every year, usually around 15k miles. i use the transmission flusher at my school, and use a flusher fluid for that (crappy chevron dexron/mercon from costco). i then put the car up on a rack and drop the pan and drain the converter (i have a drain plug in my grand marquis' torque converter). and change the filter and push fluid both ways through the cooler lines to flush it out. i then fill it with a good synthetic, motorcraft or mobil 1. i usually leave the flush fluid in for a day or 2. im weird when i work on my cars, but it seems to work great for me.