There are many ways to work the bag and its all defined by what you want to accomplish.
You can throw punches,kicks ,elbows to experiment with angles,distance and timing.Or work on your footwork=entry,lateral evasion,getting out,etc.
Your can power-punch to build up power or try to throw as many punches/kicks in a minute/round to build power-endurance.
Here is one method I used effectively to build my hitting power.
Lets say you want to increase the power in your left high kick.You would throw 10 left high kicks as fast as possible.Then,you take a few breaths,relax for a few seconds.Then,you throw 3 single left high kicks as hard as possible.
Thats one set.
Rest for a minute or two,and repaeat the set.keep repeating sets until you reach the peak of power.