I agree with a lot of the answers other mothers have given you, J.. But I would also like to present two small tidbits that my mother used when I was small. I remember being 5 or 6, and when I would get upset, my mother would tell me to go wash my face. This was my cue to take my time, literally wash my face, and thus figuratively wash the negative/argumentative attitude away. I honestly can't tell you how I have learned to do it, but I am able now, on my own, in the middle of a heated debate, to stop, take a deep breath, and change my attitude. Perhaps you could institute something similar with your daughter. The key there is to not rush her "facewashing". The point is to give her something physical to do that takes her mind off the negative situation, and at first, that may take up to 10 or 15 minutes (try to get her to use cold water.) :D :D
Something else that my mother realized as I got older was the music I was listening to. I have no idea what, if anything, your daughter listens to on the radio, but I was raised on 50s oldies, and what was popular in the 80s, but then I became a preteen in the mid-90s and began to listen to the more popular, somewhat angrier, music. I think it took my mother a while to realize it, but when she did, she literally told me I could only listen to oldies, the happy 50s stuff again. And while I didn't WANT to admit it, at 8 or 9 years old, I really DID feel a difference after a week or two. Just another little detail to notice in your daughter's life. :D
I wish you the BEST of luck (from a VERY happy person now!)
K.