Kicking gives any fighter an advantage when it comes to combat sports such as mixed martial arts.  Kicks tend to have more power generated behind then than boxing techniques because of the difference in size with the muscle groups.  You are able to cover more distance from you and your opponent than you can with punching.  But it is not enough that you can swing your leg at your opponent.  Along with having great technique, you should have great amounts of speed and power behind your kicks, otherwise they would be useless if you are up against a stronger opponent.
The workout routine below will help you develop speed and power within your kicks.  After at least a month of doing this drill, you will condition your legs so you will not be as easily tired.  Before starting this workout, it is important that you understand and comprehend proper technique.  This workout is geared towards those who understand and have learned and developed proper technique.  This will prevent any bad habits later on which become harder to break.
The workout is broken down to technique/exercise and time/repetitions.
Front Kick

• Slow Kick – 1 minute: Do not set down.
• Moderate Pace Front Kick – 1 minute
• Explosive / Fast Front Kick – 1 minute

Perform each drill with the same leg.  Once you are finished, start over with the other leg.
Squats

• Squat – 1 minute
• Lunge, Right Leg – 1 minute
• Lunge, Left Leg – 1 minute

Perform each squat and lunge at a slow to moderate pace.
Round Kick

• Slow Kick – 1 minute: Do not set down.
• Moderate Pace Round Kick – 1 minute
• Explosive / Fast Round Kick – 1 minute

Perform each drill with the same leg.  Once you are finished, start over with the other leg.
Calf Raises

• Toes pointed straight – 1 minute
• Toes pointed outward – 1 minute
• Toes pointed inward – 1 minute

Side Kicks

• Slow Kick – 1 minute: Do not set down.
• Moderate Kick – 1 minute
• Fast Kick – 1 minute.

Perform each drill with the same leg.  After you are finished, start the other leg.
Finale

• Wall sit – 1 minute
• Horse Stance holding a kettle ball – 1 minute
• Squats with dumbbells – 1 minute

Remember to stretch thoroughly before and after your workout.  Warm up at the beginning and cool down at the end.
Conclusion
I hope this workout challenged you and is helping you develop your kicking techniques further than you have before.  Kicking is very important in combat or self defense because of its range and power potential.  Do not stop learning new ways to make your kicks more powerful and more explosive.  It can only help you later.