I was taught some very effective exercises by a Grand Master here in Colorado to improve all your kicking.
He was a TKD/HKD master and while I study Chuan Fa, I still make these drills part of my daily workout.
Basic Principle worth mentioning: Always practice kicking higher then you actually intend to use. However, since TKD has so many kicks the head, get ready to kick the moon, man.
1. Leg Strength Drill - Set up a folding chair. Kick over the high back of the chair and hold for 15 secs. Do 20 reps on each side.
- Front
- Side
- Round
- Back
(later ad crescents, ape kicks, etc.)
Up your reps weekly.
2. Moving Target Drill- Have someone at your school use the hand kick shield and give you target drills. Use all of your kicks to practice efficent movement to the target. Start slow and speed up.
3. Bag Drill - First, begin with rotating through your kicks (20 reps each side) with as much power as possible. Then - Unload on the bag for 2 minutes, then rest for 2. Repeat 10 times.
Then, if you're still not tired...
4. Setup several bags in a zig-zag path. Unload on one for 10 secnonds, run to the next and do the same. When you reach the end of the path, sprint back to the beginning and do it all over again.
Do this at least three times a week and you'll notice huge gains in your leg strength and sparring ability. However, I should stress the need for more modern TKD students to study hands and effective (i.e. non sport) applications. There would be nothing worse then getting in a fight, thinking you can do it and then realizing the laws of the dojang do not apply. It could be as easy as practicing Basic Buddhapalm. Best of luck with your training