"I always thought my son would marry his ex-girlfriend!"
This is a less-than-subtle way of your mother-in-law letting you know she wishes her son had married his ex-girlfriend. The easiest way to let her know that her unpleasantness won’t be tolerated is to reply, "My ex-boyfriend’s mother was super nice to me." If you don’t want to be quite that blatant in your response, you can say, "Well I’m glad he didn’t because then you wouldn’t have become my mother-in-law." Leave it up to her to decide if you are being sincere or sarcastic.
"Be sure to wash his shirts inside-out to preserve their color."
Few things are more vexing to a new bride than to have Mumsy-in-law share her little helpful hints. Women take this sort of input as, "I’ve been taking care of him his whole life, and I know how to do it best." Mothers would do well to remember that it really doesn’t matter how household chores get done, as long as they get done. Or don’t get done. In the big scheme of things, does it really matter? Is it really anyone’s business? As the wife, you need to make it abundantly clear that domestic tutoring is not welcome. A simple, "I’ll be sure to tell him that when he does his own laundry," should do the trick.
"Your hair looks nice dear. Much better than yesterday."
If your mother-in-law would give Marie from Everybody Loves Raymond a run for her money, you’re going to have to sharpen your forked tongue. Appearance critiques are probably the most unwelcome of all the in-law feedback. If you want to be really snarky, put it right back on Marie. "That shirt you’re wearing looks newer (cleaner, less tight — insert insult here) than the one you wore yesterday." But if you really want to put her in her place without hating yourself later, simply say, "I know. Some days I get so busy caring for your grandkids, I don’t have time to do my hair." That should shut her up.
"I’ll make little Junior’s snack. I know what children like."
Nothing will turn a perfectly sane woman into an eye-clawing hell cat faster than criticizing her parenting. And if that parenting "advice" comes from the mouth of the woman who gave birth to her husband? It’s the human equivalent of separating a mother bear from her cubs. Not pretty. Not pretty at all.
A super snarky response sounds a little something like this, "That’s weird because (my husband) said he always had to make his own snacks growing up." If you really want to hit intruder-in-law where it hurts, bring up one of her parental shortcomings that your husband has shared with you.
If you want to be nicer about it, you can say, "Thanks for the help. I seem to do most of the work around here." That will suggest she did a crappy job of raising her son without saying, "You did a crappy job raising your son. He’s a total Momma’s boy."
More on mothers-in-law
How to handle the mother-in-law from hell
4 Tactics for handling parenting criticism from the in-laws
Your relationship with your in-laws