Hello Billman,
I have a background in gymnastics choreography and physical therapy...most people call me "The Posture Lady." Here are a few helpful hints for posture during ballroom dancing (and day to day activities!):
1) Gently pull your shoulder blade down and back and in (you will be using the Lower trapezius muscle). Do this OFTEN while you sit and stand. Your shoulder blades should only be about 4 inches apart from each other...not 8 to 10 inches because of slouching.
2) Try pulling your right shoulder blade down and back and in as you lift your right elbow up to the side. Do this in front of a mirror and avoid shrugging your shoulder. If your right shoulder starts to elevate, stop and start over. Fix your shoulder blade first and then try lifting again. Do 5x per day.
3) Stand with your back against a wall. Pull your lower abdominals UP and IN so that your low back touches the wall. This will give you a solid "center" to start with. (Relax your shoulders though.) Place your bent elbows out to the side and slide your arms up above your head...keeping your low back and abs flat and avoiding shoulder shrugging. Relax your arms back down to your sides. Repeat 3 to 5x per day. Stop any exercise if you have any pain and consult a medical professional if needed.
*Try these three suggestions and best of luck with ballroom posture!
theposturelady@aol.com

What odd creatures ballroom dancers are. The legs start up on the ribcage. The arms start at the shoulder blades. And the spine doesn't stop at the neck , it goes straight up through the skull which only allows it to turn from one side to the other, and not backwards or foreward or over sideways.

hi Posture-lady
Many thanks for this useful advice. I will try it out.
I have a couple of questions (i'm sure i'll have more when i've tried your exercises a few times!)
1)I'm not sure i could slide my hands up above my head without my shoulders shrugging a little. I've tried watching my upper body posture when on weights machines in the gym, and when my upper arms go above horizontal, the shoulders start to come up too. Isn't this normal? and unavoidable.
2)The 'keeping low back against the wall' exercise. Do you mean the whole of the back from coccyx region, through the waist, up to the shoulder blade area?. I thought there was a natural curve, which would keep the waist a little away from the wall. i know dancers are meant to have nice flat backs (the men anyway!) but i didn't think it was necessary to go this far.
Anyway, i'll try your exercises, which might help my long-standing (though slowly improving) problem

Phil. We spent a little time in a technique class only this last week on how the ladies can take that arch out of the back, which in Latin is an absolute no. It was all done through the stomach muscles. Not an excercise. Ladies have a tendancy to have a curved spine more than the man. This was a Latin technique class and it was demonstrated both ways so we could see for ourselves what it did, and can look like.

Hi posture lady,
thanks for the advice,
i had a class few days back, and my instructor had the same things to say.
I'm male and 180 cms tall, bad posture had always been my problem.
i'm slowly working on it and my dancing is improving.
i always used to wonder why i would go offbalance and bad posture is the first and right place to start with,