Because I'm not yet a mother, I have to show SOME restraint in lecturing people about their parenting!
I think the same will hold true of lecturing people who have kids older than mine in the future. However, being a developmental psychologist and doing a lot of reading on family issues gives me SOME right to opinions, I feel.
The thing I try to ALWAYS do is to support parents who are doing things "right". This comes up either in a conversation where a "right" parent and a "wrong" parent (or other opinion-giver) are at odds, or when someone talking to me says, "Can you believe X is using parenting technique Y?!" Then I will talk about the advantages of that technique and the benefits I observe in the child.
When I'm only talking to someone "wrong", I almost always express my views at least a little, but how long I continue depends on their reaction. For example, a former co-worker was all excited about BF while pregnant, but when her baby was 3mo and she came back to work she said she'd weaned him a month earlier. "Really! Why??" "Oh, I was just so over it." "How do you mean?" "I was so over it," she repeated firmly. That told me she didn't want to discuss it, so I dropped it. (I still wonder....) When a "wrong" parent seems receptive to the "right" idea or uncertain about the "wrong" idea, I really go to town with advocacy, unless they start flinching!
Because most of the people I hang around with are non-parents, most of the parenting discussions are theoretical. There are two kinds of those: "What I'm going to do when I have kids," and "What all parents ought to be doing, dammit!" (Funny how the latter so often comes from people who are purposely "child-free".
: ) In these discussions, because there's no specific child being discussed and because I have more experience w/children than most of the people present, my opinions generally get a decent hearing, and people are pretty receptive. The one that really works like a charm is when people are talking about how to get kids to behave properly in public, usually advocating setting a lot of rules and enforcing them punitively....I explain that one of a child's strongest desires is to do what bigger people do and be accepted into our world, so instead of making rules about "what you must not do and what will happen if you do it" explain "this is what we do"; positive reinforcement is more effective than negative. I love seeing the lightbulbs go on in my friends' eyes!