Heavy bag work should be limited to several times per weak. In each training session, stop and move to another exercise when you become so tired that form and technique fails. If you continue to hit the bag at this point you will run a high risk of unnecessary injury. Move to shadow boxing when your tired.

Art

Art is really the expression of the self. The more complicated and restricted the method, the less opportunity for the expression of one's original sense of freedom.
                                                                                            ~ Bruce Lee

Its that balance between natural instinct and disciplined control.

Meditations
. Alcohol,(beer in this case) can be used to relax before training. Just a small amount does the trick, especially  for someone with lower than .5 percent body fat.
 Its like a potion. A little bit can be of benefit at times. Too much can be a hindrance.
~A little bit can relax you enough that you may avoid injury that you could have possibly suffered due to tightness.
~Too much could cause injury due to reckless technique.
Meditate on these things.