I have the exact same problem. Instead of having my stirrup on the ball of my foot, I have them on my toes. So, on Tuesday my trainer put my stirrups further back told me to push my heels out. She was explaining how she finds that some people respond more to a different way of wording it. The three different ways she says it is "Heels out", "Toes in" and "Knees in" (without gripping). Another thing that helped me was sitting up and really bringing my elbows back.
I have problems keeping my heels down and out when I'm cantering (particularily on my new mare). So, she told me to slide my stirrups further back, heels out, and almost take my leg completely off of my mare's sides.
I hope that made sense and good luck!
"You know, for as long as I can remember, I've had memories." ~Colin Mochrie