I feel your pain. My 6 and a half year old daughter is the same way. I'm a broken record: "In this house, we are kind to each other. I am waiting to hear your kind words." And I wait for her to speak in kind voice, a sincere voice. I find if I can manage to stay calm and keep my own emotions in check (hard!) it all goes much better. LIKE I HAVE TO BE KIND TOO. :) I often need to clarify the difference between using discipline and being a "mean mommy".
I also find the preemptive approach helpful. "Yes, you may play dress-up before dinner if you can promise when I say it is time to clean up and help with setting the table you will do so quickly and with a good attitude."
And if this doesn't work and she's sassy anyway, I calmly say that I'm very sorry but there will be no story at bedtime. "You must be too tired or you wouldn't be behaving this way. You usually have such golden manners." And I stick by it, that really clears things up for a couple of days.
Humor is effective... it can catch them off guard when they are grouchy and put things right quickly as in "Hey whoa, who are you and what have you done with my beautiful, kind child? I must find her (grabbing and shaking her) alright, where is she...have you stuffed her in a closet?" "Mom! MOM! it's me!" "Don't lie to me, have you fed her to a dragon...!" ETC.
Acting whacky lets off steam for me and often diffuses a tired, common and trying issue like sassy behavior. I get sick of hearing myself sometimes, you know?
Remember you are fighting the good fight and it's worth it!