Often with students in the youngish range (and sometimes even older ones), while they're facing the barre in first position, I'll first pick up the arches of their feet, then I'll take their thighs in my hands, try rotating them, and then jiggle them, with a lilt in my voice, ask, "Am I'm going to feel jello or muscles?!" If they respond right, you can feel their muscles firm up right away. For some, it takes a few tries but everyone can do this.
And as soon as I feel their muscles tighten, I say "Good!" and then take my hand and gently "remind" their tummies to be in, backs long (you can simply take your fingers and "roll" them up their backs to their necks -- you can see the lengthening happening; it's wonderfully amazing), make sure heads are level, pick up their arches [again], check thighs [again] -- and then say "Now, don't forget to breathe!"
There is a lot to check with each student but as they "get" a correct stance and posture, it's definitely worth it. I've found too that once they do get this, then all I have to do is spot check them -- little reminders that can eventually become mostly hands-off and even non-verbal such as my looking at someone, pointing to my own elbow and then lifting it, or taking my fingers and knitting my own rib cage together, or pointing at someone's feet and say "arches" -- they know exactly what to check and how to fix it (sometimes, yes, with a little help and fine-tuning).
And as we know, it takes a whole lot longer to put this into print or even to verbally explain it than the quick few seconds it takes to actually implement.