I think the story at the end about speaking to your child with conviction and the right tone is spot on.

I have a 20 month old daughter. I constantly struggle with this because I want to be nice even when saying 'no', but it doesn't take long to realize that being stern without being scary is the best balance. I realized that my 'no' when she opens the pantry door is totally without conviction and she doesn't respond at all. But my 'no' when she strays off the sidewalk and near the street is obviously much more stern and she knows I'm serious. The most difficult thing is saying 'no' with conviction when you just don't really care that much (like the pantry door).