From FoxyRoxy "personally when I do a shoulder in I start it off a circle and make sure to weight my inside seatbone (the inside seatbone is the direction ur wanting to travel) just make sure not to lean as ur doing this when doing any lateral movement think of going forward first and then over second bc you can't do any type of lateral movement with out forwardness. Make sure to only practice a couple of steps at a time and move to something else and then come back. "
If you are going down the long side with shoulders in the left (the horse's shoulders come leftward to the inside of the arena while the hind legs stay on the rail) then the inside of your horse is the LEFT seatbone.
However, this is NOT the direction you are travelling. You are literally travelling to the horse's right because he is crossing his left front over his right, toward his right. Thus, you are sitting on the inside seat bone, but it is not the direction he is travelling.
Which brings up an interesting point. In the book "Twisted Truths of Modern Dressage" Phillipe Karl states that the ride should deifinitely and alway sit on the seatbone that it in the horse's direction of travel, and in the case of the shoulder in , this is the OUTSIDE seatbone.
This is a big contraversy for dressage riders. I have no personal opinion on the matter, but I can say that that book is fascinating! And he gives lengthy and detailed reasoning for this position.
While interesting, it is written from a very closed minded perspective. There are a lot of "always" and "nevers" written in it, and as we all know the horse is a living and breathing thing, so we must be able to adapt our riding to best train the horse. While idealically, yes we will have our weight on the outside seatbone (towards the "driving" leg), this is not always a reality in training as the horse may become unbalanced and the rider is not always so co-ordinated when he is first learning. It is far easier to train the movement first with the rider sitting towards the direction of bend, and when more bend and expression are required, simply shift the weight over the other seatbone.