The heavy bag is one of the most commonly used training devices in boxing, or any fighting sport. There are lots of different ways to utilize the heavy bag, so i'm going to go over a few of them.
Don't try to kill the bag, a lot of people the first time they hit a bag as hard as they can with their whole body behind it. This is a good way to hurt yourself, and will not help you at all so stop trying to impress people and use proper technique. Use a proper fighting stance and punch with average strength. Make sure you review proper punching technique and make sure that you keep your wrists straight so your forearm is directly above your hand. Punching the bag with too much of a bend in the risk is a very common way to hurt your wrist, maybe even severely especially if you are thowing harder shots.
The heavy bag is great for fine tuning punches and combinations. Practice hooks, straights, and most importantly jabs as well as combinations of the three. Circle around the bag so the swinging levels out, and to give you a good moving target from different angles.
Bob and weave as if you were in an actual fight, there is no point in just working on punches and not getting used to moving around as you are throwing, and you need to get used to the physical strain it will put on your body to be constantly moving in the ring.
Work in intervals of two minutes to start out with because this will probably be all you can take if you are just getting started and not used to bag training. Work your way up to three or four minute rounds. You can use a stopwatch, kitchen timer, or your ipod/cd player to time yourself. Do three rounds or so with, no longer than two minutes between rounds.
As you work on the heavy bag your punches will get faster, stronger, and easier to throw. You will find yourself being able to last longer and longer. Like with anything, the more work you put in, the more results you will see.
