Both are translated as "for" and I have a list of twelve uses for each, which I cannot hold in my head while speaking. The list approach means I am translating rather than thinking in Spanish.
Is there an underlying idea that generates the uses, rather than twelve separate rules?
Accepted Answer
There is one, and it is roughly cause against purpose. Por looks backwards at what produced something, para looks forwards at where it is going.
Lo hice por ti: I did it because of you, you were the reason. Lo hice para ti: I did it for you, you are the destination.
Most of the list falls out of that. Por covers exchange, duration, cause, and movement through, all of which look back or look around. Para covers recipient, deadline, destination and purpose, all of which point ahead.
Motion makes it clearest. Voy por el parque is through the park, para el parque is towards it. One is the route, the other is the goal.
One thing to be wary of: the cause and purpose rule is a good generator and it will not survive every fixed expression.
Por favor, por supuesto, por fin, para siempre, para colmo. These are set phrases and trying to derive them wastes time you could spend using them.
Learn the principle for the productive cases, and take the fixed phrases as vocabulary rather than grammar. Most disagreement between learners comes from arguing about expressions that were never generated by the rule in the first place.
Gracias por is the one I had to memorise separately. It fits the rule once you see the thanks as caused by what was done, but it never felt forward looking to me at first.
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