Well, really this is about the crosses and kiza. Three quick notes:
A while back, a teacher, hanshi, 8-dan, mentioned that the arrow will generally go where the line of your shoulders is pointing, or if it does not, then it means one or more of the crosses is not correct. I took that to heart and after a while discovered that my thumb and the string were not in proper alignment. Fixing that one little thing has had a big effect, though I’m still fighting with controlling the actual alignment of my shoulders. But the point is that these little things really do matter, and they’re well worth lingering over.
Along the same lines, if I understoond correctly, another teacher (kyoshi, 7-dan) recommended shooting from a kneeling position for a while. Not tsukubai, but just rising up onto your knees from kiza, and then shooting without standing up. She said this helped with two things. One was to eliminate twist in the body, so my left shoulder. The other is that, since that doesn’t leave a lot of space between the motohazu (bottom tip) of the bow and the floor, it can help you eliminate the dropping of the hands/arms at hanare.
Finally if you’ve been reading this a while you know I’ve had a long, multi-year battle with kiza, in particular an uncontrollable sliding forward that, after a time, I would inevitably crash down on the floor. But while I was watching the All-Japan Tournament in Ise in 2013 I noticed that some people were sitting with a much greater angle between their thighs and the floor, so I decided to try this. It means rising up, putting extra distance between your heels and your buttocks, and doing this requires a lot of endurance in the quadriceps. But it does work. No more sliding forward. So while kiza is never one of my favourite things to do, I’m pretty much OK now, both at shinsa and in sharei. If you have the same troubles, give this a try. I think it may be particularly applicable to people like me, with long legs (or at least long femurs). You will need to build up endurance in those muscles, but that’s just a matter of practice (or you can go to a gym that has those exercise machines).