Tucker's problems during shoulder-in and haunches-in stem from his burning desire to over-achieve (I have no idea how I raised him this way, it's not like I sat in the front row and raised my hand my whole life or anything...). Specifically, he wants to give me way too much angle, which then defeats the purpose because I lose the bend and the hind end engagement and we don't really do what I want the shoulder-in or haunches-in to accomplish.
Starting in the corner of the ring, with the longside of the arena ahead of you, trot a small circle (around 10m for you dressag-y types, I suppose), and then shoulder-in coming out of the circle as you get to the rail. Keep the shoulder in for about 5-6 steps, then straighten the horse out again and trot forward, diagonally, toward the quarter line. You want to trot forward at the same angle that you had during the shoulder-in. Then get your bend and ask for the shoulder in again, and repeat the exercise. So, the path you're taking down the longside of the arena looks like this:
I didn't make this up obviously and I'm sure some of you have seen it before, but I wanted to share because I found it very effective. While we didn't always get the shoulder-in I wanted during the first try, the second one was very, very good every time, which means we must be doing something right. I think it's a combination of me concentrating on the angle I want because I know what direction I want to head when I go forward diagonally, and Tucker understanding the concept a little more clearly.
Hope you find this useful! We sure did!