Sometimes approaching things backwards works as well - look for the shoulder that is very clearly BACK as opposed to forward, and line up your diagonals accordingly. Sometimes it's easier to detect the back shoulder as opposed to the forward one.
While riding the Clyde in my avatar photo his mane is so unbelievably long that it literally covers his right shoulder most of the time, so trying to see the diagonal when on left reign was tough. I got into the habit of just looking at his left shoulder instead and getting my diagonals that way.
It all comes with time.