We've found it easier to cut in half the recommended service intreval on transmission fluid is easier than installing aftermarket coolers and can give you an earlier indication of problems, or one forming.

Research and find out what gears are actually overdrive (in some newer manuals, 4th as well as 5th gear), less than 1:1, and avoid using them under load (up any grade). Same is true for automatics, be careful of the lockup to overdrive, if you notice the transmission shifting in and out of overdrive excessively, turn off the "O/D" or shift in to 3rd or Drive.