"Sit deep in the saddle and push her into a canter."
"Keep your horse from stargazing!"
"Stop sitting on the pommel!" This is when my riding instructer tells us to stand without stirrups. LOL
"Lift your horse over the jump." Although that isn't really correct...
This is always to prevent us from falling. "If you fall, you owe me 2 litres of coke!"
"Don't let her run away from you!"