(sorry, i'm way late posting this!)
The simplest way to know when to use one or the other is that nang (long) always has several meanings like time, manner, measure or intensity (it is both a preposition and an adverb), while ng (short) is mainly a conjunction with the English equivalents of “of,” “by” and “from.”
The new “Ortograpiyang Pambansa,” the basic orthographic reform manual on Filipino spelling and usage being advocated by the Komisyon ng Wikang Filipino (KWF), devotes a special section to nang and ng.
As the language authority, here are its examples of usage, which are definite examples of clarity.
First, according to the “Ortograpiya,” use nang as a synonym for noong. For example: “Umaga nang barilin si Rizal. Nang umagang iyon ay lumubha ang sakit ni Pedro.”
Note: the author posted this erratum in the next installment: "In the eighth paragraph of Language Matters (Part 3), the labeling of the parts of speech was interchanged by the author. It should have read: 'The simplest way to know when to use one or the other is that nang (long) always has several meanings like time, manner, measure or intensity (it is both a conjunction and an adverb), while ng (short) is mainly a preposition with the English equivalents of “of,” “by” and “from.” ' "
Read more:
http://newsinfo.inquirer.net/459277/similar-in-sound-different-in-sense