If the primary address uses all available characters, the secondary unit designator should be on the first address line – preceding the primary address. The unit designator should never be on the line following the primary address. Remember, the post office reads an address from the bottom up.
Example:
Mr. Michael Murray Apt C 5800 Springfield Gardens Cir Springfield VA 22152-1058
Always use the USPS standard abbreviations for secondary unit designators as follows:
http://pe.usps.gov/cpim/ftp/pubs/pub28/pub28.pdf
Abbreviations p. 41, Last Word Abbreviations p. 46, Street p. 57, Business Word p. 79
Avoid using an attention line if possible. If necessary, use only an office or title rather than a name to prevent obsolescence.
Always abbreviate Post Office Box as PO Box.
Place the PO Box immediately above the last line if a mailing file contains both a physical address and a PO Box. Do not enter data into the second address line until data has been entered into the first address line.
Example: Mr. John Doe
1201 Broad St
PO Box 1001
Falls Church VA 22042-2102
http://pe.usps.com/text/dmm300/602.htm#xd 602 1 4
There are style manuals that deal with formal addressing style. There are several U.S. Postal Service publications dealing with addressing. The most comprehensive and accessible publication is Postal Addressing Standards, Publication 28. This publication is available from the U.S. Postal Service as well as in a portable document format (PDF) on the World Wide Web at:
-The US Postal Service National Customer Support Center has a very useful web site that provides extensive address and zip code information: http://pe.usps.gov/cpim/ftp/pubs/pub28/pub28.pdf;
http://www.usps.gov/ncsc/