A few things: Change your fluid as often as possible. The main reason that a transmission fails is heat. Transmissions get really hot with bad fluid dirty fluid, and the worst thing is once a fluid has been overheated, it loses all the good properties that make ATF useful. Even if it gets overheated ONCE. I'll try to find hte link about ATF fluid temperatures and ATF fluid life. I always use wheel ramps to change my fluids, and when I drain the ATF I usually have to add about 5-6.5 quarts. I don't jack up one side, just use the wheel ramps.

Also, make sure when you're refilling that you put the transmission in R, N, D, and 3,2,1 (the last three just to be safe, because I don't think it would actually be any different than D, but it's just superstition I guess).