Shadowboxing and heavy bag drills are some of the best tools to develop your fighting skill if you are training alone.  You are able to work on sparring strategy, movement, and on combinations.  It does not hurt to increase your stamina and build endurance.
Hitting the bag and striking the air are much more than just punches and kicks.  They offer you the ability to hone your skill and improve weak areas in your technique.  Boxers and martial artists use these tools as part of their training without fail.
In the section below, you will find an example training workout to add to or modify your current routine. It is a routine that I found quite intense, but also quite enjoyable and rejuvenating.
Warm Up
First, we need to start our warm up before we jump into training.  Training is not complete without it. Follow the format below and do as many sets as you can based on how much time you have.  Please be aware: go at your own pace, and do not over train.

• 30 Jumping Jacks
• 5 Pushups
• 25 High Knee or Running in Place
• 7 Burpees
• 30 Jumping Jacks
• 7 Burpees
• 25 High Knee or Running in Place
• 5 Pushups
• 30 Jumping Jacks

Shadowboxing
After your warm up, take ample time to stretch.  After you have done that, jump into the next phase of training.  During the shadow boxing segment, focus on strategy, movement, and angles.  Think about what you are weak at in your sparring matches and think on how you could improve.  Throw combinations, slip, evade, and do not forget to move.
Heavy Bag Drills
When you get to the heavy bag drills, focus on power, technique, and combinations.  The kicker to this, no pun intended, is to continuously hit the bag for the allotted time frame.  Small pauses are acceptable, but try not to break for an extended amount of time.  One combination I would recommend is the Jab, Jab, Cross, Hook, Back Leg Round Kick.  This combination was fun to work on while adding it to other combinations.
Rounds and Sets
Start out by shadowboxing for 3 minutes, followed  by a 15 second rest.  Then, continue onto the Heavy Bag drill followed by a 30 second rest.  Perform 10 rounds at a minimum, and go for as many as you can do inside your training session.
Hopefully, this workout will give you some added rejuvenation as it did me during my hiatus, and inspires some other workouts as well.  It is one workout I plan on putting into my current training regimen.