Six is a big age for 'attitude'. They've got strong emotions, but no 'filter' for expressing them more politely. I've got a highly dramatic 7 year old, and this is what we do with her:
When her voice is rude, we stop her and say "That was rude. Did you mean to say "Mom, I can't find any underwear?"" (Today's morning meltdown was because she couldn't find any underwear that was clean (it was in the dryer). "You never do any laundry! I don't have anything to wear! I can't find a shirt! Why don't you do some laundry!" ) We've done that long enough now that we can simply say "That was rude. Try again." It's pretty interesting to watch because she can phrase it more politely, but she still can't control her tone of voice. Right now, I'm letting that slide because I don't think that she CAN control the tone of voice at 7. We're working on taking a deep breath before rephrasing.
Having a very clear routine for things really helps my dd. She's usually OK doing her chores if we do them at the 'regular' time (which for us is right after dinner). If we delay them to just before bedtime, it's a disaster. She also needs regular meals, regular down-time, and regular large motor outside time. Life is a lot happier when she's been outside for a good portion of the day. Life is happier when she's in a routine. (We're traveling now visiting grandparents, and she's out of her routine. She's done well for 10 days, but the travel is starting to wear her down.)
FOOD. Dd has a high need for protein. She's incredibly cranky and defiant when she's hungry. She's pretty cranky and obstinate when she's tired. Heaven help us if she's hungry and tired! We must take care of those things first. So, this morning I told her that her voice was rude, and insisted that she eat breakfast.
SLEEP. See above. We had a long day yesterday (baseball game + visit to another relative + swimming in grandma's pool until 9:30, then she stayed up late reading Harry Potter).
One-on-one time with mom. She loves dad. She craves 1-1 time with mom. So, we play stuffed animals together. Last night we played "house" in the pool. Other kids may have another favored parent, but ideally, they'd get 1-1 time with both parents. My kids are happiest when they get 20-30 minutes or so a day of my undivided attention, where they lead the play. Several of my favorite parenting books recommend this (Playful Parenting by Larry Cohen and The Challenging Child by Stanley Greenspan). It works. Ds (age 10) chooses to toss a baseball around or play soccer. Dd (age 7) chooses imaginary play with stuffed animals. She even bought me my own stuffed animal for my birthday because then I would have to make it talk.
Finally, when all else fails, or when w'ere at the end of our rope, we send her to her room until she can be civil. A line I've been repeating a lot these days is "You are free to have your feelings. You are not free to make the rest of the family miserable while you scream and carry on." I usually give her a hug and we snuggle for a bit when she comes back.
I also think that if she's got some behaviors that are driving you crazy or that are inappropriate, it helps to prioritize them. What is she doing that's not appropriate? What would you like her to do in those circumstances? How can you teach her those skills? Right now, we're working on dd's emotional outbursts when life does not go according to her plan. It's a long road. Dd does not respond to rewards, and removing privileges actually makes the situation worse. Yelling doesn't help, but it is my natural reaction sometimes.