Even after taking 4 years of college Spanish and living abroad, I still don't have a very firm control of when to use por or para. What are the basic rules on when to use either.
From here (with minor corrections by me):
para indicates direction or intention
por indicates origin or cause
A Spanish teacher explained the difference with an "intention arrow". In the case of 'para', the intention moves from the subject to the direct object:
In the case of 'por', the intention moves from the direct object to the subject. This requires a little more imagination and liberal interpretation of "intention."
To put the Por or Para question into something easier to remember, I have put the reasons (taken from Puntos de Parttida Spanish Textbook) in a visual style similar to Flimzy's arrows of intention, but with a few additions on the Por side.
Por is represented by a backwards arrow to represent the following meanings:
Por is represented by a circle with an arrow running through it for the following meanings:
Por is represented by a recycle symbol for the following meanings:
Para is represented by a forward pointing arrow:
(One meaning of para that doesn't fit with the forward arrow is "in comparison with others in the group." I don't have a picture for that one.
If anyone knows how to put visuals in here to help with this explanation, please let me know. It would look better with the circles and arrows actually drawn in. I'm very new to this.
I know this is old but I'd like to add my answer as well. The lists have been taken from a grammar textbook but most examples (except the last two) are not just pasted... sometimes I needed some help with the structure or ideas, though. :) So if you spot a mistake, feel free to correct.