My mother's classic is "Now I don't mean this as a criticism, but..."
I never had the nerve to stop her right there, but the urge was always with me. My reaction was to just avoid her when she got like that (go home shortly afterwards and not see her for a while). A little passive aggressive, but conflict was avoided so I didn't get branded the "mean offspring" by my family.
Once I gave birth to THE GRANDDAUGHTER*, mom really cut back on the criticism. I never used my daughter as a pawn, but mom realizes that I avoid her when she annoys me and thus won't bring my kids over as much. She doesn't want to risk not seeing Tater Tot as much. So far, that fear has kept her criticism in check.
* She admits to being more comfortable with Tater Tot (my DD) than with Small Fry (my DS) and Junior Monkey (my DNeph) because she only had girls. My sisters and I are on the watch for any visible favoritism to squash it, but so far she's been pretty equal with all the kids. So please don't flame me for allowing my kids around a grandparent with favorites--if she ever makes Small Fry feel less than his sister, I will confront her--and to heck with a label of being "mean". However, since he is only 2 months old, it's not an issue yet.