Don't post stuff you don't want other people to see - plain and simple. Someone could just as easily have copied and re-uploaded it. Hope this topic doesn't devolve into Facebook-bashing.
I'll also make sure to avoid showing up in any pictures taken by a camera I don't control. And stop everyone I know or don't know that may have taken a picture of me and posted it from doing so. It's naive to think this is only stuff posted by the people in the pictures.
If you don't want your photo taken, or the possibility of it showing online, don't go outside!