A.B.
I don't know if this is good advice... as I worry we are not doing the right thing... but it gets the result we want....
To this day at 22 months, from the very first time we tried, saying "no" sternly causes my daughter to laugh and repeat the action... whatever it is. I am not really sure how to resolve that problem.... but rather then try and try and fail... i just tried a different tactic.
Rather than say "NO", I just say her name and tell her to touch gently and it hurts mommy when she hits. Really any action that warrents a "NO", I replace with a quiet, gentle explanation and I ask her to say sorry. She always stops.... she really doesn't hit anymore at all. She understands what gentle touching is (especially because we showed her early on how to pet the cat).
So, I never say "no". I just physically stop her and tell her what to do instead. I am afraid it is not exactly "discpline" but my daughter is very well behaved.