They interfere
"It's hard for grandparents not to give advice on child rearing because they care so much about the kids," points out Susie Kohl, author of The Best Things Parents Do and a mother-in-law herself. But some advice isn't welcome -- and may be dead wrong. If your in-laws tell you to spank a child who's throwing a tantrum, just state your position directly: "We think time-outs work really well with Evan." Then let it go; you don't need to convert your in-laws to your position. When Kohl herself oversteps, her daughter-in-law says, "Don't worry, I can handle it," and then changes the subject.
But sometimes you can't just blow it off. One California mom was outraged when her in-laws decided to give her 3-year-old "speech therapy" (which they weren't qualified to do in the first place) when they were babysitting. "They kept telling me that David talks funny, and I kept telling them that his pediatrician wasn't worried," says the mom. "But one day when I got to their house to pick up my son, he was talking with his tongue stuck to the roof of his mouth. He was self-conscious about his speech for a long time after that."
When in-laws overstep their bounds like that, let them know how angry and upset you are, but soften the message by saying, "I know you're trying to help, but this is a sensitive issue and we'd like to handle it ourselves." Your in-laws need to know that you are the parent: They had their shot at raising kids, and now they have no choice but to follow your lead, whether they agree or not.

They criticize your decisions

Sometimes in-law comments are nothing more than thinly disguised disapproval. One Michigan mother says, "My mother-in-law kept saying how wonderful it was that my kids loved their nanny so much, and how lucky I was that she could be with them while I worked such long hours!"
A Minnesota mother meets with much more open criticism every time her husband stays home from work to care for a sick child -- despite the fact that his job is flexible and he loves the one-on-one time. "They think I'm being a bad wife and a bad mother," she says.
It's best to take the straightforward, honest approach, says Newman. Tell them you don't appreciate being judged and held to their impossible standards. At least they're put on notice and made aware of your feelings. And although you may not be able to change their minds, for sanity's sake, it can't hurt to bring in some outside authority to shore up your position -- like an article you read, say, on why having a hands-on dad is good for kids.

They don't support your family rules
If your kids see their grandparents only a few times a year, this may be one of those things you compromise about: It won't hurt a child to stay up late or watch an extra hour of TV every now and then. But if your kids spend a lot of time at Grandma's, better step in. One Alabama grandmother lives three doors down, and the kids were constantly bringing home goody bags full of candy until their mother, who'd tried every other method of getting her message across, finally mentioned that she'd be sending all dental bills to Grandma from that point on.
Usually it doesn't take such extreme measures to get grandparents to comply. A simple explanation and the offer of an alternative ought to do the trick: "Jenna's such a picky eater that I hate for her to fill up on cookies. I'm going to bring over some cheese or raisins." Grandparents just want to make the kids happy; your job is to give them the tools so they don't (or won't) break obvious family rules.