Go look in a mirror and practice this. Hold onto a doorway and bend your knees a little, and check your posture in that mirror. You don't even need a full length mirror - your bathroom is fine, because you're only looking at your shoulders. If you feel wobbly, try to use your stomach and back muscles a little more. If you can't find those muscles, go do a couple stomach crunches to show you where they are!
Got that posture figured out? Now here's the tricky bit. While doing your footwork*, isolate your shoulders and don't let them rotate forward & back/left & right as your hips move. Hold them steady. Don't let them wiggle and flop around and distract from your nice turnout you've worked on. Pay extra close attention when you rotate to the right - keep that left shoulder in place!
Okay, here's your inspiration clips - you've earned it!
"Chool Song", Dean Collins and Jewel McGowan dancing
In her switches at 1:00, Jewel holds her top half up and steady, and her shoulders barely wiggle - even with those giant sleeves that accentuate everything! Notice too that she opens up from Dean a little - she's not perfectly parallel to him (which makes this a better angle for the audience), but she still holds steady.
"One Girl and Two Boys" from Swing Fever, Jean Veloz dancing with Don Gallagher and Lenny Smith
In this clip, Jean does some sassy ups and downs with her shoulders, but still keeps them back and doesn't let them rock in as she swivels. Notice too how her arm does different stylings, but she still keeps her posture.
If you feel inclined, go back and watch the last clips I posted, and this time watch their shoulders. I'm trying to post a variety of clips (and not just Buck Privates every time!), but if you're into this, go watch them all again!
*Footwork- still planning a post on this, but that one will take a little more time to write, so I'm stalling! :)