My version of "run tall" is "pretend you're riding a pony". Your hips and legs are the pony, and your torso is riding it, and it works for me as a cue to remember that my torso should stay tall and upright.

Also when I get really tired, it helps me dissociate myself from the feeling in my legs because instead of thinking "ow my legs hurt", I can think "thank you, pony, for being so strong and working so hard for me!"