Here's what I've done for years with no probs on my 89, which has synthetic trans fluid, regular filter/fluid changes, the gm upshift kit stuff, and a cooler i put on in addition to modified engine cooling:

From a stop: do whatever you want. I usually launch in 1st or 2nd and hold 2nd to 70 then 3rd to whatever.

At speed - I shift between 3 and 4 with reckless abandon - of course you don't want to hold 3rd TOO high, but I never go over 90 or so anyway.

Downshifting at speed: Just never ever shift down past 3rd (also known as to first or second) at speed. If you're not going TOO fast yet and you REALLY want to win I'd say do second sometimes but keep it rare.

And that's it. Now that I've written that, I'm sure my trans will die in short order.