The following strategy is a great one to use when you're trying to avoid getting into a power struggle with your child when enforcing limits. It still allows you to be able to respond to there misbehavior, but at a later time. It goes something like this:
Parent: "Susie, I need you to clean up your room."
Susie: "I'll do it later"
Parent: "We had agreed you would do it before lunch."
Susie: "Just leave me alone. I have to do all the work around here."
Parent: "If it is not done by lunch, I will owe you a "No" Then walk away
Now Suzy may or may not do what you asked her to do. But if she does not clean her room, the next time she asks you to do something for her later that day or the next, it will go something like this:
Susie: "Can I get a ride over to Justine's house?"
Parent: "I would've liked to have given you a ride, but if you remember you didn't clean your room when I asked so I owe you "no". Hopefully that can change after you clean your room." Then walk away.
This puts you in control of what you can control. It will also have your child starting to question the value of their oppositional behavior, particularly if they do not know when it will affect them next.