Hah, some of your comments crack me up. Yes, I've actually considered the high heels thing. Never done it though cause they hurt like crazy!
I realize my heels are way far down in that image. Like I said, that's not usual (it was during a long gallop to the next fence, and Roxy was quite strong. The frames around that one shows my heel in a more correct position, but they're a little fuzzy so I picked a clear one that you can see my stirrup in.) I do have extremely hyper-flexible ankles. I've ALWAYS had a really deep heel (well, not quite that deep... but definitely down) and my leg has always been very strong. I've never had issues with the stirrups sliding before, even with consistently deep heels. I actually have been having issues with not keeping my heels deep enough lately as a result of riding slow horses that needed quite a bit of, ahem, encouragement.
I did some work today that I think may have been somewhat beneficial, but it's still a big issue. I alternated between standing with flat heels, deep heels, and my heels UP (like wearing heels) in two point at the walk and trot. It was generally okay, until I sat back down or posted the trot; then they still slid around. I tried keeping my weight more on the ball of my foot (I put the stirrup a little past where it should be, as in the stirrup pad was like a half inch too close to my heel) which helped but again, as soon as I posted or cantered they slid. I think it's because I'm not keeping my weight in the correct place consistently? They slide the most at the posting trot and canter, where you naturally bounce a little more. I also lengthened my stirrup a hole today which helped a tad, but definitely nothing drastic.
I attached a picture from a week or two ago that shows my more typical heel position (again, ignore my stiff upper body. My riding has, obviously, been a complete wreck lately.) Just so you all don't think I'm crazy and always ride with heels like that