The essence of foot work is to avoid your opponent's attach by stepping back, then step in and counterattack. If you just keep stepping back, you will be felled. Always bear in mind that the step back is a prelude to attack : straight back, then straight forward.
Once you can do this smoothly, learn to move diagonally backwards, move sideways and move backwards in a circle until it's second nature. You must also be able to use the in-step in a straight line or circular motion in every direction from the step back position. Furthermore, you have to be able to punch and kick with every step you take.
One way of training to develop this ability is to use a stairway. Punch as you step forward, as you move down a step, and as you move up a step. Train in such a way that the energy and spring used in stepping up and down the steps improves the power of your punches and kicks. Practice this in the Southpaw stance as well.