Coach John Walker above teaches and demonstrates drills that can be used develop better transition speed.  These drills are good for changing direction, acceleration and deceleration.  

As an athlete, you must posses the ability to change speeds without wasting energy and steps.  Athletes, especially football players must be very good at moving in defined space and that most often requires the athletes to change speed and direction based on what they are seeing.  As a defensive back, if you can make sure that you have efficient use of steps and energy, that can be the difference between and intercepted pass or a completed pass by the receiver.  

Work these cone drills on your own time to help you develop better transition speed.  You can do these drills 2-3 times a week to help with your foot speed.  Work with a partner to alternate turns and to give you some recovery time.