I havent taken a lesson in a good while but I remember my instructor when I was a kid yelling across the ring "get those heels down.....are your heels down?"
I have heard my daughters instructor say "Who is running this show, you or your horse?" It helps her to remember to pay attention since she is only 10.
My favorites are 'pain is weakness leaving the body' my old trainer used to make me say that when I would have to go around 20 times in 2pt or with no stirrups or both. He would make me say it over & over
Him - 'what is pain ?'
Me- 'weakness leaving the body'
I also like 'now we're cookin' with wesson oil' haha