When asking a horse to just stand still and relax I find the following two phrases useful:
Stand like a farmer not a model
This helps people to ground themselves and form a stable base with their legs a little akimbo. I always felt justified using this phrase as I once worked with a very beautiful model and, like a dancer, she just couldn't help standing in a gorgeous pose whenever we stood still, one leg slightly in front of the other to enhance her profile. Of course she could also stand like a farmer not a model and did a great job with her horse (some people have everything!). Of course, all of my clients were beautiful!
Put on your Madonna bra
For horses that had a tendency to invade their owner's space and push them around, I would suggest that they imagine they were wearing Madonna's bra, a conical arrangement, which helped them to stand a little more assertively. The rest of the get up wasn't necessary.
