Lifting my left heel is something I do at times if I think to try and keep it down it will mess up my swing. Here is something that I have been doing that is helping me to keep my heel on the ground without thinking, it may not be for everyone and you have to be a bit cautious when doing this move.
In my daily workout routine on of the things I do is to take two 10 pound dumbbell weights and hold them together using both hands. With the weights held together in front of your body begin to turn at the waist with your feel planted firmly turn at the waist with the weights extended at about mid chest high. Turn as far as you can to the right and then to the left do as many reps as you can. While doing this do not lift your heels of the floor it is quite difficult to do without lifting your heels but once you get the routine down I have found it carries over to the golf swing.
As I said this may not be for everyone and as I said use caution when swinging the weights I hold on to the large end of the weights with both hands if you are doing them in the house it could be a disaster.
Give it a try sometime it may help some of you out there about lifting your heel plus it will give you some additional flexibility after you have don this for several weeks in a row. This is part of my daily weight routine now.