I've taught Rayne to cue with a head tilt to "fix" herself if she sits crooked or is a little forged on her automatic sit. Did you ever use a wall or short barrier when first teaching the heel position? My trainer just put a couple 2x4 pieces of wood together and laid in on the ground, so that it wasn't tall enough for her to lean on but provided a subtle barrier to keep herself straight.
We would start against the board, then take a step forward (still against the board), she would automatically sit, with the board there it would be straight and I would mark it, repeat that until we reached the end of the board. Then I would take one step past the end (no barrier now) and if she still sat straight I would mark it big. And just build from there. Some dont like using barriers or props, but it worked for me
I can't really remember now exactly how I taught her to fix herself (she swings her butt back and a little behind me). I think she just eventually figured out that she got many more rewards when she would swing that butt back and sort of picked it up on her own. Because I would tilt my head to the side to check to see if she was straight, and if she wasn't I would tell her "nope, free" and set up again. If she was straight the next time, mark and reward.
So the head title turned into a cue to fix her butt, which has actually been a great tool to use for her left turns (an idea I got from the Michael Ellis heeling DVD). Because I can title my head just as I'm making a left turn and she will swing her back end really nicely. I'll eventually fade out the head cue for the left turns though.
One of the things that really helped Rayne's heeling was teaching her rear-end awareness, something I completely skipped and then figured out I really needed it. A good thread that helped me out: