The reader sees evidence of physical changes in President Snow, especially in the final moments of the novel.  Katniss' last private meeting with Snow depicts him as being "well groomed...but weighted down with manacles" and his skin was a "pale, sickly green" (355). 

Snow's good health becomes a metaphor for the well-being of the Capitol and Panem.  When the Capitol flourishes at the beginning of The Hunger Games, Snow is vibrant and powerful.  It is only as the districts begin to rebel that the reader sees signs of Snow's deteriorating health, the smell of blood on his breath in Catching Fire.

At the end of Mockingjay, Snow's deterioration becomes complete.  The Capitol has fallen, and upon Snow's execution, the reader sees a final glimpse of him with a coughing with a "bloody dribble" on his chin (371).

Despite his failing health, one aspect of Snow does not change--his master-minded ability to manipulate and deceive.  He remains a consummate manipulator throughout all three novels, up until his triumph at seeing Katniss assassinate Coin.