Here is a clip of Marvellous Marv:http://www.youtube.com/watch?v=AJvS9GIBWGc&search=hagler
Watch for the fifth round. Notice how he spends most of his time in an orthodox stance. It the third and fourth round, he spent most of time in a southpaw stance.
At the start of the fifth, when he catches Ray with a left hook, notice that as he throws the hook, his leg follows his punch, i.e. his left leg moves from the back to the front. Marv often changed stance in this way, by throwing a punch then letting his leg come round with it.
Try it at home. Start in an orthodox stance, and throw a left jab, straight right combo. Keep on your toes. Then throw the same combo again, only this time as you throw the right, move your right foot into the lead position.
The other thing I use to do for footwork is the Cardinal point exercise. Start in an orthodox stance, keep on your toes, and imagine where you are facing is NORTH on a compass. Then bounce round 90 degrees clockwise, so you would now be facing EAST. Try and make sure when you land, you land in your stance, that you don't have to adjust your feet to get your stance again.
Now go another 90 degrees clockwise to face SOUTH, and again to face WEST. When you get to WEST, move 90 degrees -counter-clockwise to face SOUTH, but this time as you bounce round, try to land in a southpaw stance. Go counter-clockwise again to face EAST, and again, land in a southpaw stance.
Do this exercise a couple of times. Its very good for footwork, switch-hitting, and balance. It may seem silly at first, but Olympic Squads still use it to this day.
Sorry if it is hard to understand the exercises from my description. Any questions let me know.