Improving your techniques and skills

Speed, Agility, and Explosiveness:

Take short sprints, changing directions within the sprints.
Practice footwork combinations (i.e.: switch feet, stepping/skipping forward/backward, side stepping, etc). Eventually add kicks into the footwork. Begin slowly with a focus on developing proper form, and then gradually increase speed.
Bunjee cord work. (These cords can be ordered from martial arts catalogs.) Attach one end to your ankle and the other to a stable structure (a piece of heavy furniture, perhaps). There should be slight tension on the cord before you begin your technique. From there, practice your basic kicks with the added resistance from the cord. Do at least 15 repetitions, and then take off the cord and practice the same kicks without it.

Power:

Bunjee cord work on kicking bag, focusing on power.
Weight lifting with a concentration on the legs.
Squats, lunges, leg lifts, abdominal work, etc.
Stair climbing.

Range:

Stretch to increase flexibility.
Adapt techniques for long range (i.e.: drop kick, back kick).
Practice "moving in" footwork with long-range kicking combinations.
Force yourself to throw long-range techniques on the kicking bag by standing far away.

Endurance
(Do one of these activities before or after your normal workout):

Take a slow 30 minute round on the bag, picking up the pace when there are two minutes left, and going all out when there are 30 seconds left.
Jog at least a mile (or as far as you can), then do short sprints with a two-minute walk in between for the next 30 minutes.
Jump rope for 5 minutes, then take a 5-minute round on the bag. Repeat the pattern for 30 minutes (with no rest in between).

Sparring Strategy:

Spar.
Watch other people spar.