For me the trick isn't actually to think 'heels down' because I have a bad tendency that I share with many others to then push my entire foot and leg forward. I have to think of pushing my heels back towards the horse's back feet. Imagining it like that helped me a lot. Make sure your stirrups are the right length too, if they're too long your legs will wobble like crazy because you're reaching for them rather than resting your feet on them.
I LOVELOVELOVE bareback and stirrupless work! It's great for getting your leg down where it should be and for improving balance so that you don't toss your legs about trying to balance with the stirrups instead of your seat.
Someone once suggested bailing twine, I tried it it didn't work. But Pumpkinzmybaby22's sister keeps throwing her hands up when she kicks Chinga in her lesson so I've threatened many times to tie her hands to the saddle with bailing twine....hehehe