Quick explanation:
The prepositions por and para tend to be troublesome for non-native Spanish speakers. In some cases this is due to the fact that each of them is sometimes equivalent to the English preposition for. In other cases the difficulty comes from the diverse uses that each one has. For example:
Yo no sé si María tiene el dinero para comprar los boletos. - I don't know if Mary has the money to buy the tickets.
María va a comprar un regalo para ellos. - María is going to buy a present for them.
Estoy aquí por asuntos de negocios. - I'm here for business issues.
Estoy aquí por dos semanas. - I'm here for two weeks.

"In order to" and the expression of purpose or use

Para sacar buenas notas hay que estudiar.
To get good grades you have to study.

Estos zapatos son buenos para bailar.
These shoes are good for dancing.

To indicate destination (location or recipient)

Voy para Oaxaca.
I am going to Oaxaca.

Salimos para la tienda a las doce.
We left for the store at twelve.

Estos zapatos son para Miguel.
These shoes are for Miguel.

To convey the time limit (deadline) of a particular action

Tendrás que terminar para el lunes.
You'll have to finish by Monday.

Quiero las fotos para las tres.
I want the pictures by three.

considering (that)

Para alguien tan inteligente, lo que hizo es mediocre.
For (considering that) somebody as smart as he is, what he did was mediocre.

To express suitability

Los vestidos son para las mujeres.
Dresses are for women.

Este tornillo es para mi bicicleta.
This screw is for my bike.

To express the idea of "in my opinion"

Para mí, el carro verde es mejor.
In my opinion, the green car is better.

Para Luis, el rojo es el máximo.
In Luis's opinion, the red one is the best.

To express the reason or motive for an action

Lo hice por tí.
I did it for you.

Cancelaron el partido por la lluvia.
They cancelled the game because of the rain.

Austin es conocido por su música.
Austin is known for its music.

To indicate a place where movement is occurring (through)

Los ladrones entraron por la ventana.
The burglars came in through the window.

Vamos por la calle sexta.
We're going by way of Sixth Street.

Periods of time: parts of the day

por la mañana - in the morning
por la tarde - in the afternoon
por la noche - in the evening
por la noche - at night
por la madrugada - in the early morning

Por = per

Estoy contigo cien por ciento.
I'm with you one hundred percent.

Íbamos a 80 millas por hora.
We were going 80 miles per hour.

Gano diez dólares por hora.
I make ten dollars per hour.

To express price or an exchange

Te doy mi reloj por tu bici.
I'll give you my watch for your bike.

Lo compré por quince dólares.
I bought it for fifteen dollars.

Por + manner or means of transportation

Vine por avión.
I came by plane.

Fuimos a Veracruz por autobús.
We went to Veracruz by bus.

agent of passive voice Por = by

Esa canción fue escrita por Tracy Chapman.
That song was written by Tracy Chapman.

the object of an errand with verbs like: ir, enviar, mandar, venir, etc.

Mi tía vino por el pastel.
My aunt came for the cake.

La señora mandó al niño por el canasto.
The lady sent the kid to get the basket.

with "estar" to express "to be about"

Mi papá está por explotar.
My dad is about to explode.