When you take a step backwards your feet are at a diagonal. Try this one exercise as it has helped me tremendously.
* Stand by a counter, table or between two highback chairs to keep you from falling or ask your partner to spot you. Stand with your feet hip width apart with one foot in front (moreso to the side) of the other. Your feet will look diagonal and hip width apart. Distribute your weigth evenly on your feet. Focus on something at eye level while slowly turning your head left, right. Keep doing this very slowly until you get your balance, one minute should be enough.
* Focus your eye on that spot and move head up and down slowly. You will be able to increase the speed of your head turns when you get your balance.
* More difficult: Extend your arm at eye level holding your thumb up, moving your thumb to the right (a few inches) your head moves to the left (a few inches) all the while you are focused on your thumb.
* Then move your thumb to the left while head turns to the right. This, of course is very small movements.
* Then try up and down while looking at your thumb, your head goes up while your thumb goes down. Remember slow, very slow in the beginning increasing speed when you are balanced remember the thumb movements are small.
* Extend your arm and thumb up and to the right. Gazing at your thumb move it from up to the right to down left (diagonal movement across the body) one minute, repeat on other side.
* Standing with your feet diagonal look quickly to the right and back to center, then quickly to the left and back to center for one minute, then up, center, down, center one minute, then diagonal up right, down left, one minute, switch up left down, right one minute.
* Advanced movements: stand on a couch cusion while performing these. This will similate soft surfaces such as carpeting, grass, sand.
You'll feel this in your feet, ankles and legs. Good strengthening exercise too. Stronger muscles, better balance.
Hope these help you.