We said a few things about por and para a few days ago. Most of the times, explanations about these prepositions look nice and clear but it all becomes confusing when you start using them in conversation.
Out of all of the uses these prepositions have, there are two that probably are the most confusing ones:
Para expresses a purpose, while por a cause, a reason.
Let’s see a two examples:

• Llamaré por teléfono a Carlos ______ pedirle mis cosas.
• ______ levantarte tarde, perdiste el tren.

Which sentence expresses a purpose? Which one a cause? “pedirle mis cosas” is the purpose of the call. Getting up late made me (caused) lose the train.
Now, let´s think of them logically, in terms of time: Temporally, the purpose (not the enunciation of the purpose but its actual fulfillment) comes always after the main action. First, I call Carlos, after that, I will ask for my things. Here, we will always use PARA.
The cause always precedes the main action. I woke up late and only then, for that reason, I lost my train. In this case we will use POR.
Voy a Chile para estudiar español. I go to Chile to learn Spanish. I first go there then, I will study Spanish. The purpose will actually take place after I arrive in Chile.
Estoy triste por lo que le pasó a Miguel. I feel sad for what happened to Miguel. Temporally, something happened to him first and then I feel bad. But the cause takes place first.

No introduction needed. This is a big problem for any Spanish learner. When do I use por? When do I use para? And if you already know the “uses” of these prepositions, how can I use them correctly and spontaneously in conversation without making a 5 minute pause to think which one goes in my sentence (and end up picking the wrong one)?
Without going into a lot of detail, here’s what para expreses:

• Scope or recipient of the action. Estos zapatos son para tí.
• Opinion. Para mí, eso no es importante.
• With verbs that express movement, it indicates direction or destination. El tren para Veracruz sale a las 4.
• With a time value, it expresses the term or limit for an action to take place. It can also mean hasta. El trabajo debe estar terminado para el jueves que viene.

And por indicates:

• The cause or reason of an action. Lo han condenado por homicidio.
• The agent of the passive voice. El fenómeno será estudiado por especialistas.
• Approximate location, when talking about space. Tus libros están por allí.
• With verbs that express movement, indicates transit. Llegaré tarde pues de regreso pasaré por el supermercado.
• With a time value, it can indicate approximate time, periodicity or part of the day. Por las noches saco a pasear a mi perro.
• The means or instrument with which something is done. Te enviaré el documento por fax.
• Price. Este libro lo he comprado por cuarenta pesos.

This is not a complete list of the uses of these prepositions. It only covers the most frequent uses of them.
Perfect, but you know you mastered your exercises, and you still make mistakes when you speak. Well… there’s no final solution for that. Or, maybe there’s something you can try: practice. Not mere repetition, authentic practice. This is valid not only for this, but for everything. Without real practice there is no language learning.
Speaking of por vs para, regardless of the specific uses we just mentioned, I would think of them in terms of two general ideas: Para, generally speaking, will normally express a direction, an intention, while por is clearly more focused on the idea of cause and means.

I know, this is not very clear nor easy to incorporate, but once you start getting the ideas behind these to words, once you hear others use them correctly (and paying attention to how people who speaks well is a great strategy for learning a foreign language) everything will become more natural.