That's tricky, because you want to emotionally respond, but that gets down to their level. My kids have done this, often times jsut to get a rise out of me and see what works and what won't work.  My wife and I decided that the best course of action is to ignore it.  "If you're not gonna speak to me with respect, then I'm not going to answer."  You just need to find out what it is that's causing the sass.  Might be a bigger problem. 

Oof-- no one likes a sassy kid! I have a twin brother and growing up, he was always the tough one for my parents: talking back, getting into trouble, pushing buttons, etc. My parents continually compared his behavior to mine and I think that was the COMPLETELY WRONG way to go about it. I would suggest talking to your kid about where this attitiude comes from! Then go from there-- Good luck :)

Talking back is always difficult to deal with. I'd suggest discussing with your child where his new attitude is coming from. Once you find the root of the issue, you can work through it with your child so that the talking back doesn't continue. 

Have you thought about talking to your kid's teachers? They might have some insight from their time with him at school. 

I am dealing with this at home, too. What I've been doing is making her aware of her behavior. She has said she "didn't know" what she said was rude or talking back, so now when she does it I tell her and then ask her if she could rephrase it in a better way. When it comes to muttering under her breath, I send her to her room to chill out and come see me when she can be respectful.We speak sarcasm frequently in my home, so it's tough for her to see the difference. Mostly I try to keep emotion out of it and try to understand where she's coming from. It's no fun, though. For sure.

When my preteen went through this I thought about how her closest friends spoke to their parents and friends. I had a talk with her and reminded her that even though she hears her friends speaking that way, it is not okay in our house. I also paid attention to the kinds of TV shows she watched and noticed which ones had teens being sassy. Good luck and try and keep the lines of comminication open!

For me, what has worked is I bring attention to their disrespect and request they "try again". Something along the lines of, "Please re-evaluate what you have said, and speak to me in a respectful manner." If they continue being disrespectful or argumentative, we simply say, "Let's reconnect when you figure it out" and carry on with my own duties. So many people think this behavior is cute. I have one friend who has said, "She's snarky like me!" Parenting is a tough gig and allowing behavior like this now, is only asking for frustration later.

Disrespect and totally ignoring me are two of the biggest issues with my 9 year old right now.

You are not alone!  I'm following this topic for advice myself.  My six year old is scaring me how much she acts like a tween/teen — so moody already!  And the sassyness! I'm terrified for when the teen years really do hit!  Hang in there, mama!

My 8 year old can do this and I usually just let him know it is unacceptable and if he does it again he is going to his room. Then I explain why it is unacceptable and ask him how he would like to be spoken to like that.