Is there a rule for choosing when to use "por" or when to use "para" in a sentence?

*Por*:TMTT Time Money or unit of measure Transportation Through, along or by *PARA*:IDDI Intended for Destination Deadline-for a point in the future In order to

[quo]*Quote from * hedoguy Is there a rule for choosing when to use "por" or when to use "para" in a sentence?[/quo] Many of the explanations I've read about when to use para vs. por seemed rather confusing. _Por_ was said to define cause or motivation, while _para_ was said to define goal or purpose. I had a hard time seeing much difference between cause, motivation, goal, and purpose. But then I came upon this example in a book entitled _Spanish Now!_ by Ruth Silverstein et al. It shows how the question "Why do you work so much?" can mean something different depending on whether you use para or por: QUESTION: ¿*Para* qué trabajas tanto? Why (for what purpose) do you work so much? RESPONSE: Trabajo *para* tener dinero. I work in order to have money. QUESTION: ¿*Por* qué trabajas tanto? Why (for what reason, cause) do you work so much? RESPONSE: Trabajo *por* no tener bastante dinero. I work on account of not having enough money.

http://spanish.about.com/cs/grammar/a/porpara.htm This is the most thorough I've read on the subject thus far.