If I can see sozlet, I can usually understand her. And she usually knows enough to make sure I can see her before saying anything. But yeah, a lot of it is not just what she says but how she says it (I guess that's kind of the definition of sass). What I've been having a hard time with is when she acts like a 13-year-old in front of her friends (been happening more often lately, I know she got the idea from at least one friend). Minor things, mostly. But, like, I served the girls a snack, including some freshly washed grapes. Sozlet usually likes grapes. Her bf made some comment (I didn't see it) and then sozlet made a little "take it back" gesture to me and sneered, "these grapes are NASTY."

(That's when I got close and reminded her about time-outs, though it wasn't that bad in and of itself.)
Ummm, still thinking of solutions, still not coming up with much beyond a time-out.
I know just what Green Witch means, my grandma didn't do exactly that but I can so imagine her doing so. Guilt is a tricky one though, can be handled well especially with a big dose of humor but hard to pull off without creating landmines.
Something else I've done (also landmine-y) is that if she is acting annoying I'll just, like, go sit down with a book. She'll be like, hey, I was talking to you, and I'll say well, I'd rather talk to you when you're being civil. Then I read for a while either she goes off and plays for a while (which is fine) and then comes back when she's ready to be civil, or she changes her mind right away. Or she has a chicken fit. That one happens occasionally but if it does there's usually something else going on with her and so I do The Talk for that, too. ("Is something wrong? What's going on?") That seems to be another way of sending a message that what she's doing is unacceptable.