Hi C.,
Negativism, or the often-heard "no!" response is extremely common in toddlers and preschoolers, so congratulations, your daughter is right on target for a little self assertion. Here are a few ideas that have worked for me. (You will notice that I am going to sidestep Time Outs and punishments or "consequences" and going to go directly to some techniques that won't get you hung up in that moment. This is to keep your day moving along and to keep your daughter and yourself positively focused!)
1. WHENEVER POSSIBLE, USE NON-VERBAL METHODS OF DIRECTION. By this, I mean that often we engage our children in a lot of talk, and a lot of opportunities to say no. Using our bodies to direct via hand signals changes the dynamic a bit. For example, when my son stands on his chair, I use a hand gesture of my pointer finger going down, much like a conductor of an orchestra. At this point, he sits down, or if he doesn't, I walk over and place him on the floor and just give him the facts "You may sit on the chair or stand on the floor." Which leads me to-
2. GIVE THE CHILD TWO POSITIVE CHOICES THAT WORK FOR YOU. When giving directions, offer a choice whenever possible, even if it's the smallest of choices. "You may sit next to me or stand next to me while we wait" offers the child a way to go beyond deciding "yes or no" and instead helps them focus on the task at hand. Having a choice of walking or riding, of washing hands with bar soap or soft soap... the guiding idea here for us is to decide what is most important and what needs to be done, then giving our child a chance to choose how they want to do it. Be flexible too--sometimes our kids will come up with great ideas of "how to do it" themselves which are perfectly acceptable. This also models tolerance and shows our kids that it's all right for different people to have their own way of doing things.
3. AVOID GETTING STUCK IN A CYCLE OF NAGGING. That is, if you need to give a verbal direction or cue,("It's time for bed. Let's go get your pajamas"), do not give it more than twice before taking a deep, calm breath and physically assisting the child to help you in what needs to be done. This isn't always easy, but focusing on the task and not your daughter's staunch "no" does two things--it moves the activity along and let's her know you mean business without engaging in a power struggle. Scooping up a child and suggesting something positive "Let's go see what pajamas are in your drawer. Hmmm...I see star pj's and snowman pj's. Which do you want to wear?" This sort of refocusing helps your child physically move along to the next step.
4. AVOID PROTRACTED DISCUSSIONS OF "NO" WHENEVER YOU CAN. Some people will disagree with this, but the best method of countering negativity is ignoring it. I don't believe in having big "let's examine our behavior" discussions with young children, partially because the child is not always capable of being constructive with it (other methods like scripting and coaching in the moment are far more effective)and because it gives that "no" lots of attention, which only encourages and reinforces its repetition. Parents will often want to either punish children who "answer back", and while some parents would consider the child to be "answering back" and defiant, this is actually an important developmental step for the child to separate from their parent and become their own person, and is critical to the development of their self. One has to decide if we are raising children to be our acquiesent puppets, or their own empowered, competent person who can think for themselves. We adults have to be mindful that their "No" has nothing to do with us, and everything to do with the child asserting their selves.
one more thing...
GIVE A YES WHENEVER YOU CAN AND AVOID "NO" WHENEVER POSSIBLE. Does your child hear a lot of No from adults? Our own negativity is often reflected in our children's speech. If you give a "yes" whenever you can, it really helps. This doesn't mean giving our child what they want all the time, but telling them "yes" and when in the future they might get what they want *does* avoid quite a bit of trouble. This amazingly works for kids of all ages. "Yes, you may have a cookie, after naptime" or "Yes, we can go to the library, tomorrow" or "Yes, we will visit the zoo the next time Grandma comes" are all ways of giving some context as to when the activity or desired item will be coming their way. As kids are wont to do, younger ones will often forget about what they wanted, satisfied with their positive response, and move on.
I can't think of any other brilliant ideas, but I just want to reiterate here that punishing a child for not 'hopping to' and following orders is counterproductive both to helping our children move on and to our day as a whole. Keeping in mind that our physical guidance helps so much more than engaging in distractions like Time Out, which are confusing ("I didn't come when mama called me to dinner, so now I have to sit in a chair and still not eat dinner"), and that this isn't about us, we can move through our day more pleasantly than trying to teach our kids a lesson. They'll learn eventually. They will also keep saying no to us, throughout their childhood and as they mature to being adults. Giving them good context to practice their autonomy, like allowing them to say "no thanks" to a serving or peas or bubbles in the bath, etc. also gives our kids a little breathing room. Sure, tantrums sometimes develop too, but this is also pretty par for the course with young ones, and they do eventually grow out of it!