It's funny you would mention this. This was a big problem and still is on occassion. When I used to work in daycare, we were not allowed to say "NO" to the children. We also were not supposed to say anything "negative" to the children. So, what in the world do you say when they are doing something that they are not suppose to do???????
"I'm sorry, but what you are doing is dangerous. You need to do something else"!! If a child bit another, we could not say "NO BITING" but instead, "we use our teeth for food, you hurt my friend and it made her sad".
My point is that when I got the job, my son was not even two yet and another one was on the way. Working in daycare (even for just a little while, not only helped me better deal with having two children, but also retrained me before things got carried away (ie. NO). You just have to "re-train" yourself to stop using the word "NO". If your DD is using the "NO" word (ie, "please get in the shower?" "NO") then you need to tell her "when you say that it really makes me sad". Also, stop giving her "YES and NO" questions. This worked with my son so that when I asked him to do something, there was not a choice of "NO". (ie, "do you want red or green?", "it's time for you to get in the shower so we can get ready for bed") I found that with my son, 5 and now my daughter, 2, If I give them a "choice" I need to respect their answer. If I don't want to hear "no" as the answer, I don't ask them "yes or no" questions. (You need to get into the shower so we can get ready for bed....NO..... I didn't ASK you if you wanted to, I told you we need to get this done, please get in the shower, now) {instead of "would you like to take a bath?"}
Kids get plenty of chances for choices, but they also need to learn that sometimes, we don't get a choice.
It does take a while to train them to not say "no" anymore, but if you are vigilant about changing the way you say things and take it out of your vocabulary as much as possible, if they don't hear it, they wont say it (usually).
Also, when my son tells me something in an UGLY way, I stop him and tell him "what is a different way you could tell me to do that?" and he will say "mommy, can you please not do that". Now, he even catches himself and will stop and re-phrase the way he says something on his own.
Good luck, I know it's not easy.