Some time ago, I stumbled across a free 29 page e-book for learning state postal abbreviations.
It includes maps, worksheets and flashcards to help your student learn the names and locations of states as well as their 2 letter postal abbreviations. It also includes lesson plans for using the materials.
Learn the States and Postal Abbreviations (PDF) from fivejs.com
One of the things that I like from this packet is a postal abbreviation study sheet which arranges the states' postal codes by how they are made (first two letters, first and last letters, etc)