Spend more on your cassettes. XX, XT, or XTR. They come on a large carrier that distributes the load better. Unfortunately, with the Shimano spline pattern on aluminum freehub bodies, them's the breaks. Knock the high ridges down with a file, so you can easily slide the new cassette on.
Shimano tried to do something about it 10 years ago, or so, but people bitched fiercely about all of their wheels being obsolete, so they canned it.