Everything I've been taught and read does indicate that the word no is overused and loses its meaning. Plus it doesn't teach them WHY they shouldn't do something.
So instead of saying no everytime zoey would try to eat the dog food at 10mos old we'd say "doggies food. lets get zoey some food" and we'd redirect her to human food.
With roman, he's a climber now at age 2 - I'd be saying 'no" all day but instead we do say things like "climbing is for the fort, not for the bookcase cuz it will fall and give you big owies".
With repeated asking and 3yo's - i have a boy I watch this age. He'll ask "may I have some chips". If the answer is no I'll usually say (to all the kids) "we aren't having chips today, we're having apple slices with our sandwhiches". If he asks again I usually ask him the question "what did I already tell you about chips" and he'll say "you said we're not having chips today" and I tell him how smart he is for remembering
Every child is different, but I think we do overuse the word no and me and dh (and teen dd) are working really hard and using other words and explaining why something is dangerous, or why we're not doing something instead of just "no" or the ultimate becoming my own mother "because I said so"
Examples.
zoey: can we make a project
me : yes, after roman goes to sleep (instead of no, not right now) because he will try to eat and throw the beads/glue etc
roman is climbing the bookshelf
me helping him down saying :bookshelves aren't for climbing, it might fall and hurt you! do you want to climb? then lets go climb outside on the fort
someone is throwing a toy
me: Toys hurt if you get hit with one so lets not throw them. If you want to throw things, lets go get a ball and we can throw the ball to each other
I mean there are a million ways to play it. I think its much easier then no, cuz 'no' sets you up for repeated asking.
child : can we go outside?
you: no
child : please can we go outside?
you: i said no
child : but i want to go outside
you: i said NO!
or....
child : can we go outside
you : after dinner when its not so hot outside we'll go out and water the garden and play in the fort but its so very hot out we'd better pick something else to do. What could we do really fun inside?
then the child can pick a different activity, kwim?
does any of this help? lol