My DD is 6 and she is the same too - sulky, stroppy when she doesn't get her way, talking back, etc...
I have started telling her that no matter the situation it is never okay to be rude or stroppy to anyone and I refuse to talk to her until she starts acting and talking respectfully. This way I leave the room (and let her know why) before I get too angry. It seems to be daily occurence though...