Remember when you are about to begin run/walk, the key here is to start off at a very slow pace and keep the stride tight/short for the first few minutes. If you take off too fast you're tendancy is to open your stride and then you're back to your old way of heel striking.
The other point about the begin run/walk is that when you are in your best posture and set up to go, talk yourself through the start i.e. say to yourself - "focus on the feet, lean from the ankles and peel up your feet behind you". This way you don't hesitate and start walking/running on unbalanced feet.