There are many drills that can be utilized to improve foot speed. The agility ladder and acceleration drills are examples of drills aimed at improving "foot speed". There are a couple things that you should keep in mind though before jumping right into drill work.
Athletic Stance- If an athlete is not positioned for efficient force application, the athlete will not move efficiently. Before starting an athlete in any type of drillwork or strength training make sure the athlete has sound movement techniques. Bill Hartman mentioned in a previous post about a pitcher improving velocity by simply adjusting technique. The same principle holds true in this case. Prior to any speed or strength work we can make our athletes faster by improving techniques.
Psychology of Movement-All athletes want to be fast but the nervous system has the final say in how the body will move. How can a coach drill for this? Make everything as random and gamelike as possible. The path goes as follows: Stimulus->Interpretation->Response. If a coach or trainer keeps the stimulus constant, a Pavlovian result will follow. The athlete will respond the same even if the stimulus changes or disappears. Sports are random in nature so the training must be as well.
You can find DVD's that outline actuall drillwork by going to
www.sportsspeedetc.com