I believe word choices matter, and I try to reflect that in my parenting. While directing a child, there is usually both a positive and negative way to say the same thing.
My earliest memory of choosing the positive phrase is calling out, "Walk!" to my little ones at the pool instead of, "Don't run!" Do you see? Same result, but one sounds better. ;-)
Goodness knows I still say no and I still lose my temper, but I hope that consciously choosing the gentler direction will have a cumulative, positive effect, or at least sound less like nagging.
Some other examples:
"Remember ______." instead of "Don't forget ______."
"Talk like a big girl." vs. "Don't talk like a baby."
"Please [do such & such]." vs. "Don't ______."
I'll bet you've got powerful phrases of your own--care to share?