I use a mity vac hand pump to pull out at least a half quart of trans fluid every motor oil change, that way I know there is fresh fluid and additives in there sure it takes some extra time but if it keeps a rebuild away for and extra 10,000 miles it is well worth it in my book. I have a dedicated hose that is as long as the dipstick and I pull the fluid out through the dipstick tube. Before summer I will pull out 2 quarts and every now and then I will do a full quart during motor oil changes!
Lubegard additives,Lubegard transmission fluid additives
Their products are the only I ever tell people to use and it actually works, My buddies girls escape was having shifting problems and I told him to dump a bottle of that in there and now it shifts like brand new, he couldn't believe it since he was ready to go ahead and bring it to the shop.