A great boxer understands the importance of strong punches and combinations to keep his opponent on his toes and to continually establish scenarios where he can land a strong punch. A punching bag is a great place to exercise combinations and build endurance. Below are a few of the most effective shots/combos out there.
Heavy Bag Shots and Combos
Jab, double jab, triple jab-: While the jab isn't the strongest punch, it plays an important role of distracting your opponent. Be sure to mix it up a bit. Don't always do the standard single jab or jab-cross. Why not jab-jab-cross or jab-jab-jab cross. As long as you mix it up, your opponent won't be able to predict you. Pick a spot on the punching bag and try to jab repeatedly hitting that same spot.
Right cross: Usually preceded by a jab, ensure that you really follow through when doing the right cross. You want it to be powerful yet quick. If it is two slow, the opponent can get a punch in over the bend in the elbow straight to your face whereas if your arm is straightened, your chin is protected by your shoulder.
Jab-right cross: The most well known combo. Throw the heavy bag a quick jab with the left and a powerful cross with the right, being sure to follow through and step out of the way after throwing the cross. The bag can still knock you over if you don't step away after a hard punch, just like an opponent can throw a counter punch.
Regard the punching bag as your opponent
After all combinations, practice rolling under the punching bag, ducking and dodging around the bag. Imagine that you just unleashed hell on you opponent, and the opponent is coming back with the counter-shot of a right hand. Strive to go under the right hand (heavy bag) and come up with a right hook. If you can dodge the punching bag, you can dodge your opponent's cross.
We don't want to just go out there and start swinging at the bag. Once we throw a combination, we should know which way to shift our weight. Again the most common way to counter a cross is to duck under the opponents arm (or the punching bag) and once under, you can take a crack at the body or chin.
After every punch, it's best to keep moving or circle the punching bag as to get out of the path of your opponent. Most people move around the bag to the left since most out there are righties. Be sure to practice this both to the left and right. If you always go the same way, it's easier for your opponent to predict, making it harder for you.
Use the heavy bag for everything it has to offer. Don't just beat the crap out of it. Pretend it's an opponent that fights back. When it swings back after you throw a cross, duck under it and counter with a hook or an uppercut. The punching bag is one of the best tools out there. Just be sure to use it right and regard it as an opponent and not just a psychological release.