The mistake that I see most often is nage giving nage his center back during the pivot.
My instructor has a couple of things that he does incredibly well. Shihonage is one of them.
If we really get in a rut, he has us lay the back of our inside wrist across our forehead and the back of the outside wrist across the pulse area of the inside wrist. The wrists should be "cupped" around your forehead.
It's a crutch, but when my shihonage gets to really sucking, I retreat to it till I get the groove back.