As you state, an aftermarket tranny cooler will save your transmission, and add many miles to it's life. And also as you mention, overdrive towing is strictly forbidden. The 700r4 is not a bad transmission, you just have to know it's limitations. Also, if you do a lot of aftermarket improvement to your engine, you'll want to beef up the 700r4, as there is a limit to the HP it is rated for. If you are towing a lot, changing the filter/fluid will also help extend the tranny life. But the cooler and not towing anything in overdrive will do more for you than anything else. You can also get aftermarket chips for your vehicle that'll modify when/how it shifts to accommodate towing as well.
Big Chris