A.D.
She is testing her limits to see how far she can go before you yell. To her, this is all a game, a form of entertainment so to speak. She pushes you because she knows she has a little more time before you really want her to obey. I recommend giving her one warning "Please pick up your toys or I will have to punish you". If she doesn't move immediately to pick up her toys, she receives her punishment- perhaps that punishment could be sitting in a chair for 5 minutes (using the age rule- one minute for each year), or the loss of a privilege, or going to bed early. Be sure to know what punishment you will use before you threaten, that way you can include that in the warning. If you feel you need to, use a timer "I am going to set the timer for 5 minutes, if your toys are not picked up when the timer beeps,(insert your punishment here)" Then be CONSISTENT. That is the key. Remember, she will not change overnight, and it won't be easy for you, but if you are faithful and consistent, she will realize that mommy & daddy are in charge and she needs to obey.
I also like the rewards system, but I don't believe it should be used alone. In addition to the discipline, it would be a great system; used alone I fear it might turn into a bribery system rather than a reward. She could earn stars for cleaning up, brushing teeth, etc., that could be her daily reward, then give her a larger goal to shoot for, after a determined number of stars, maybe a special outing to someplace she really wants to go. Just remember, at her age, it is hard for her to grasp the concept of time and it is more difficult for her to stay focused on a far off goal, so make the larger reward attainable.
I recommend the book by Dr. James Dobson "Dare to Discipline". It really helps to put discipline in perspective. Good Luck!