"Don't cross the street!"
"Stay on the sidewalk!"
Notice that I gave you two identical commands. However, I'm willing to bet one of them would be much easier for you to follow than the other. Why?
Your mind cannot perceive the absence of something.
If I say the words "Don't cross the street," you can't not think of crossing the street. Sure consciously you may somehow be able to maintain your focus and block all thoughts of street crossing from your mind, but your subconscious mind is still processing the thought of "crossing the street".
But what if I say the words "Stay on the sidewalk." Your mind wouldn't be thinking of crossing the street, your mind would be focused on staying on the sidewalk.
Next time you think of barking out a command to someone (say your child), don't tell him or her what not to do, tell him what to do. Your subconscious mind doesn't process the "no" or the "don't" or the "never", it just processes the command itself.