Most importantly, turn off the overdrive unless you're truly loafing along (level or downhill). You should learn to recognize when the transmission torque converter is in lockup and when it isn't. If it's in lockup, you're golden, but if it's not, you're rapidly building heat in the transmission. Don't trust the A/T temp light unless you're certain it actually works, many have reported that theirs does not light up when the key is first turned on.
Turning off overdrive merely locks out 4th gear. The torque converter is almost impossible to get out of lockup above 50 MPH in third gear, but will unlock at the drop of a hat when you ask for any power in 4th (ovedrive). Experimenting with the O/D Off button is the best way to figure out the difference between 3-locked, 4-unlocked, and 4-locked. If you ask for moderate power in 4th and the RPMs jump, you've probably just caused the TC to unlock. If you immediately turn the overdrive off and the RPMs jump again, the first "shift" was not a kickdown to third, the second was. Note the RPMs at various speeds and gear/lockup conditions and you'll be able to stay ahead of the game.
A transmission cooler is an inexpensive way to protect your transmission. They run about $50-70 and are easy to install and/or inexpensive to have installed. Many opt for the 24,000 lb. GVW rated units for motorhomes and such. If you live in an area where it gets really cold, make sure you install the cooler in series with and BEFORE the factory cooling circuit integrated into the radiator to prevent over-cooling of the fluid. If the aftermarket cooler over-cools, the fluid will pick up some heat from the radiator before returning to the transmission. Obviously, during hot weather, it won't "heat" the fluid since it will still be doing its job as if the aftermarket unit weren't there.
Having the fluid flushed with synthetic will also help quite a bit. It's expensive, the 15-18 quarts of fluid alone will run you about $100 on top of the cost of the service, but it's worth it. I've got 30,000 miles on my Mobil 1 ATF and it looks absolutely good as new.
The rest of the driveline could benefit somewhat from new fluids. Here's my how-to:
http://mywebpages.comcast.net/xterra...uid_change.txt
Brent