Golden Rules for Saying "No"
Only say "no" if you mean it. - This boils down to the fact that a parent needs to be thoughtful about each and every "no". It is amazingly easy to use either "yes" or "no" as a default answer, but default answers lead to situations where you either say "yes" or "no" when the other answer is the more appropriate answer. This leads to breaking rule #2...
Never back down from a "no". - Kids learn quickly whether a parent can be whined (or pestered or begged) out of a "no" answer. So if you say "no", you have to stick to your guns, especially with young children (who tend to test this more).
These rules actually help set up and enforce one of an important parent boundaries. I call it: "The Mommy said No, Do Not Ask Again or There Will Be Consequences." boundary.
Okay, there is a third rule that applies, but this one applies to every parenting situation - the United Parental Front (or UPF). But that's a different post.