H.P.
I agree that you should first recognize that they are not coming from a place of malice. They genuinely think that they are telling you the RIGHT things to do. They just don't necessarily realize that they shouldn't do it in quite that way. What I did was allow it to be a discussion, if it's something I cared to discuss.
"You think so? Well, I might look into that. So far, I've been thinking along these lines, so thanks for that perspective." And then change the subject. If they mention something that you've already decided against, and you don't mind sharing, then you can tell them, "We considered that and decided to do this instead."