I'm sympathtic, having had this happen from time to time myself. It's especially easy when working on modern right-side-rear-wheel spokes because with their high dish these tend to have ultra high tension. Here's a few hints.
1- as others have said, good spoke wrench fit is critical, or use a well made four corner spoke wench which works like a box wrench. And make sure to fully engage the flats on the nipple.
2- when building new wheels, build them over dished too far to the right, and do all the work there until the very end, where you'll use the looser left spokes to correct the dish to the left and add bring the wheel to full tension. Done right you won't need to turn those over tight right nipples.
3- a handy DIY tool for working on very tight wheels is a cut down broom handle. If you need to turn a very tight nipple, inset the handle btween the rim and hub and use it to gently lever the rim over so as to tke some tension off that spoke. (Don't get carried away, it's a good way to potato chip a wheel) With some tension relieved you're less likely to round off the nipple flats.
4- live a good life, and don't give the gods any reason to punish you.
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