What is an effective method of discipline for my very sy 8 year old?

My daughter is going through some big personality changes. My once sweet cooperative child has become very bratty and disrespectful. She whines and even hits us when she does not get her way. She is rude around company which is so embarring. She lost her manners. She often tells me to "shut up" She whines about having to do homework and drags it out for hours complaining when it can all be done in 20 minutes. She argues with me over having to brush her teeth and hair or to wash her hands. Everything has become a battle. She argues with me over every little thing. I want to ask if you can suggest what kind of consequence I should give her for not listening and arguing and also for the rude manner in which she speaks to me and her dad. Our home life is very stable. There hasn't been any major change in her life. School seems to be going well. I talk to her all the time and tell her its ok to be angry or frustrated but talk to me about what's really going on. She just seems lately to get a kick out of misbehaving. Especially when we are visiting relatives. You know how embarring it can be when your kid is acting out and all the other kids look like angels. I feel I've lost control. I'd like to know what I can do that would be effective when she doesn't listen. The problem is she misbehaves so much thoughout the day. There are at least 5 infractions a day. How can I discipline each one? Help please!!