Here's some useful information from etrailer.com:
Frequently Asked Questions About Transmission Coolers | etrailer.com
When I had my Dodge 1500 tranny rebuilt, the mechanic told me that there were 3 secrets to getting the most out of a tranny:
1. Add a tranny oil cooler
2. Service the tranny per recommended service intervals
3. If carrying much over a few hundred pounds, if towing anything, if going uphill, if going into a headwind....kick off the overdrive.
I've since put 190K on that tranny, so I'm believing he gave me great advice!!