Hey, I'm bumping this up coz I have a similar issue. Not just how you practice generally, but the best way to learn a difficult part. These days I am challenging myself by learning a tough little step jump, and I am doing it the way I have always done it - thru sheer repetition and drill. But I notice that as my wrist gets tired, the jump often gets worse, not better. Should I stop sooner? Do something else? And it is mental too - sometimes I do it best when not thinking, and then when I watch myself play it I "fudge" it up!