So what do you do?
With strangers, I don't say a word. Depending upon how offensive the advice is, I either smile (like when you eat something that doesn't taste good, but don't want to offend the chef), or I immediately turn my back and walk away. I save my best stares for those days when I'm at my worst... when the last thing I want is the lady in line behind me telling me how much sugar the candy I'm loading onto the conveyor belt has. It's that look that says, "I may or may not have a dagger in my purse, so it's best to hush." I then start a conversation with the teller or with one of my kids in order to divert any further commentary.
It's rarely a problem with friends because, well, they're friends. They know me, and know when to comment and when to bite their tongues. With acquaintances, I offer a friendly thanks and, again, change the topic. (Distraction is one my strongest virtues.) I also regularly use lines like, "I'll have to remember that," or "What a unique idea." I use the word unique even when the idea isn't; it's my passive-aggresive way of getting my point across. It may not be the most emotionally healthy response, but it makes me feel good.
Where do you draw the line when it comes to unsolicited parental advice?
My rule of thumb is that the only advice I want is the advice I receive after I ask for it. When my son began to refuse bedtime (after a routine had been set for months), I asked some of my fellow female co-workers how they enforced their bedtime routines.
When a relative admonished me for letting my son into our bed after a string of nights struggling to get him to sleep, I told her, "If the worst thing that happens to him at 15 months is that he spends a few nights sleeping with mom and dad, I'm sure he'll be fine." And then I abruptly changed the topic and asked her about work.
If you want advice, ask people you trust. If you get unwelcome guidance, do your best to walk away. But if someone crosses the line, let them know what they did. Standing up for yourself will only help make you more of the mom you want to be.