one thing i focus on while pedalling and trying to keep my heels down more is to mentally focus on the pull in your foot. i know this sounds contrary to popular belief, but hey, it works for me.

when i want to have a heels down, solid pedal stroke, i focus on pedalling in a rectangle. hear me out.

1) obviously be conscious of the angle in your ankle (i.e. heels up or down)
2) next i focus on trying to pull my cleat off the bottom of my road shoe. in doing this, i'm focusing on pulling backwards straight from 3 o'clock to 9 o'clock.
3) then i focus on again trying to rip the cleat off but by pushing my foot straight across from 9 o'clock to 3 o'clock.

[i know everybody says "pedal in circles" but if you can do this technique effectively, you actually are pedalling in a circle and also a more efficient one because there's more force being generated around the entire circle instead of mainly just the downward portions. ]

this may sound weird but hop on the bike and give it a try. if it doesn't work for you then oh well. it works for me.