I am so there! It's such an auto reaction to say NO NO. And, as you say, it's cute sometimes when they parrot it back to us. The challenge is that one night you'll lose your marbles because she won't listen to you--or worse, laugh at you--when you say "No, do not play with your nightlight when it is plugged into the wall." For us, no became a game and therefore became the downfall of our discipline strategy.
I read that if you retrain yourself to say one phrase it's easier than trying to always have something different to say. My default is "Jack, where's your XXX? This could be where is your book, your nose, your ball, your doggie, anything that's reasonably findable (now that he's learning body parts, that's an easy one). This is especially effective if he's getting ready
to do something instead of in the middle of, say, climbing on the coffee table.
Good luck with your budding little rebel!