#1 Rule in Life: Mind your own (darned) business.
If no one asks your opinion, don't give it.
If you are compelled to give advice, give it once and move on. Do not continue to give the advice if someone has already ignored it once.
Do not interfere in how another person is parenting his or her own child (imminent physical harm notwithstanding).
Judge not - lest ye be judged.
Cast not the first stone (or I might pummel you with rocks).
And those who live in bottle-feeding houses shouldn't throw stones at breastfeeders.