When I first started learning diagonals, it was really hard for me. My trainer always said, "you go up when the outside leg goes up", so I just assumed that meant the outside shoulder went up as well. It didn't actually click in my brain until I realised that the outside shoulder actually goes DOWN when that leg comes up. I'm not sure if you're having that same problem or not, but I've never had a problem with diagonals since then.
Now, I can just feel them, but unfortunately I don't know how to explain how I do that.