The US Post Office prefers City followed by 2 spaces with no comma, then the 2 letter state code, followed by 2 spaces, then the zip code. Commas and periods should not be used.

http://www.usps.com/send/preparemailandpackages/labelsandaddressing/usingthecorrectaddress.htm

Hi Kate,

The Canadian post office "prefers" no comma as well, but we believe it makes the address more readable, and it's not prohibited.