Hi HJEmma, nice to meet you.
I suddenly found this very problem with my boy last weekend, it was our first time in the ring in a group, we usually do dressage, so it's just us.
The only way I could get him to stand still, and it did have everyone including me nearly crying with laughter in the end (good job it was schooling show) was for me just to let out a huge breath in a sigh, and totally relax everything, and give him some rein.
Every time I did it, both in the ring and waiting outside, he just stretched into the contact a little and stood still as anything
You may not want to be so obvious in a line up while showing, but I would suggest practicing it out of the ring and see if it works.
Hoep someone has more advice for you, and good luck