Don’t stick your knees up in the air.
Grip with your knees and the calf of your leg.
Let your stirrups out (for most men).
Carry your legs back.
Don’t let your reins flop.
Don’t hollow out your back.
Don’t let your horse back up (unless the movement is a retrograde one).
Drive him forward.
Don’t let him bend his neck at the shoulder.
Use your leg (or legs).