The Chair Seat, it's a huge huge pet peeve of mine and unfortunately a student of mine has been riding with one for years it seems. Her previous instructors have mentioned it to her but never worked to correct it, but now she's willing (ok ok it took some nagging on my part, but she wants to show so it's got to go!).
The big issue here is she is so so stable when riding with this position. I've tried exercises without stirrups and asking her to stretch her legs down, rotate her hips to have less of an "on the seat pockets" angle, even tried the silly idea of putting the stirrups down a hole or two, since she really likes short stirrups, in hopes she'd sink weight into her heels to keep the stirrups on (loss of stirrups is a huge problem no matter the stirrup length) but it didn't work. It's pretty clear she's tense through the knee and pinches with the knee no matter what we are doing as well. She does all this bareback too!
What have you guys heard about correcting such an ingrained habit? What tricks have worked for you? I've been looking into stretches, doing warm-ups thinking about equitation at the walk and once that's better moving up to trot...I'm running out of ideas, and I hate to admit it :(