... use good quality tools that fit properly and keep it square on the fastener head.
I work in hydraulics with the many socket-head capscrews in some larger sizes. I enjoy getting friends who brag about the low cost tool set they bought and getting them to compare their wrenches vs a REAL (for me, JH Williams/SnapOn Industrial) wrench on say a 9/16 or 16mm Allen wrench in a vise. The slop will amaze you.
The other problem is in nuts from the box stores; I think they buy them specially to almost work with either metric or English wrenches.