Leith Darkin (the author of last month's superb article on grip training) discusses training to increase your punching power. Here he takes a brief look at the ideas behind the Stretch Shortening Cycle (SSC) and how various plyometric exercises are of great benefit; in addition to the basic biomechanical principles that assist in punching.
The two articles stated as pre-requisites for this one are The Stretch Shortening Cycle in throwing and striking sports and What do all Martial Arts have in common. Also well worth reading.
If you're not familiar with the boxing punch types mentioned in the above articles, there's a brief rundown (with videos) at Saddoboxing.com.