I had to have the tranny rebuilt in my '99 Ram 5.9L, at about 100K. I asked the mechanic how to make the rebuilt tranny last forever and he gave me these instructions:
1. Install a transmission cooler
2. Change the transmission fluid as recommended
3. Anytime you tow anything, or carry more than 500 lbs in the bed, turn the overdrive (guess they changed it to T/H later) off.
I followed his recommendations and that tranny ran perfectly until I totaled the truck at 318,000 miles.
That wasn't today's 8 speed transmission, but I believe those general recommendations are solid.
T