You have probably seen those fighters who seem to move so fast and unpredictable, they appear super human.  Ali as well as other great fighters of their time have the athleticsm some can only dream.  Being able to move quickly in and out of your opponents range while still being able to sting them requires fast footwork development. 
Fluid vs. Stationary Stances
Fluid movement, or, constant motion is always better than being flat-footed.  Remembering that action is always faster than reaction, a fighter in a stationary position will take much more time to respond to an attack he is not expecting, which buys the offensive fighter much more time to strike.  Fluid movement by constantly moving will give you a better chance to react quickly and move out of striking range. 
Footwork Drills to Improve Movement
Footwork requires a lot of practice.  Conditioning, Strength, and Plyometrics all play a part into developing the type of movement you want when you fight. Below is a list of exercises I use to work on fast footwork.

• Side Shuffle
• Front to Back Shuffle
• Running Backwards
• Tuck Jumps
• Bouncing on the balls of your feet
• Running in Place

Side Shuffle – Shuffling from side to side will improve lateral movement.
Front to Back Shuffle – Similar to the Side Shuffle, but you are moving forward and backward.
Running Backwards – Exactly as it means.  This helps in case you have to evade a strike.
Tuck Jumps – Increases your vertical and spring, and provides endurance conditioning. 
Bouncing – Bouncing on the balls of your feet will help strengthen your legs for the springing motion.  This will propel you much faster away from a strike.
Running in Place – Running in place helps build your cardio and endurance which is necessary for fast movement.  A lack of cardio will get you winded quickly, and you will find yourself flat-footed and a stationary target.
The exercises here are only a small list of the exhaustive lists of exercises out there today.  It is always harder to hit a moving target, so fast feet is required to keep from being tagged.