i too am going through this issue. currently i just keep practicing it in my training (not over doing it though) and i try to have the shoulders freer and looser so i can move them where i want them.
Karl Mikolka has a great exercise for getting the horse to bend in the ribs which will improve the angle in SI. He calls it Turn on the Forehand in Motion.
http://www.karlmikolka.com/Inspiration.html
This exercise is described in the first issue of his "Inspiration".
It is well worth the money to buy this! There are many more exercises as well.
Being able to do 10 m volte is very important and there are many exercises for improving SI.
I had a really good breakthrough today! I shifted my weight more to the outside (so I was probably right in the middle) and my horse did a lovely shoulder in! It just shows how often we say to them" do this" and then we don't let them!
i also watch the engagement i have coming off the corner and make sure i have a good angle at the start. free shoulders are necessary for this. if i start to lose it, i do a 10m circle get more activity and engagement and continue.
More angle per se is created by using outside hh, however that easily end up in a too straight horse as well. Remember shoulder in is developed from a circle. 20m circle ridden straight ahead is shoulder fore (2 1/2 tracks), 10 m circle is a 3 track shoulder in (on three tracks), and a 6/8m volte is four track shoulder in. There is not more angle in those exercise but increasing bend (as the circles have). In none is the angle to be greater than 30 degrees. The greater the axial rotation/use of the inside hind (w/o crossing) the greater the bend, and the horse is reactive to inside leg pulsing into the outside connect. In no variety of s.i. should the rider hang on the inside rein, the leg into the outside rein is what allows the exercise most effectively.
Restart the si several times and expiriment. Start the new circle and then hh on the outside rein. Every hh has a neutral action, horse touches the outside rein more because of the action of the inside leg, remains bended because of the outside stretching back from hip to heel. A horse cannot be stronger unless the rider is also. Ask for a step, give the inside rein. Make sure that your hips are parallel to the horses and the shoulders parallel to the horses (spiral seat), look between the horse's ears.