Two Words : One leg squats.
OKAY! That's Three Words.
Here's my technique. I do one legged squats in the moving elevator, in a pencil skirt, in flat shoes.
Yes, I DO IT ALONE!!
I found that pencil skirts worked really well in forcing me to keep from waving my free leg around to keep my balance. Instead, I'm forced to tighten up my gut and my lower calf to keep my legs in the correct position. It worked. Honestly, without that skirt forcing me to keep my legs together I couldn't have advanced as quick as I did because I would have given up and gone with an easy sloppy position. Within a week, I was doing solid sit positions down AND UP on one leg.
Why mess with a moving elevator? It introduces enough vibration to give you instability going down and up. I found I had to really work on my technique. Also the vibration makes me work harder, so I only need to do one on each ride.
I now go down on one leg with my free leg wrapped in the correct sit spin position to half way between floors, then rise up to an exit position as I reach the next floor. About half the time I rise up to the exit position (unwrap the free leg and extend it behind me as I rise) and hold the extended leg position through the elevator going through its stop. Holding an extended leg through the elevator stop really forces good balance and squeezing the calf as I rise.
I know what you're thinking: This girl is waaay too obsessed with figure skating.
Well, all I can say is, "I'm sixty-six doing one leg squats. Not bad."