Por:
1. Reason
No podemos dormir por el ruido - we can't sleep because of the noise
2. Time or locate (not specific, but more less you know the time or the place)
Hemos quedado por el centro - we stayed at the center
Volveré a mi casa por la Navedad - I am going to come back home for Christmas
3. Method of doing something
Hemos hablado por facebook - we have talked on facebook
4. Exchange or shopping
Te cambio mi perro por el tuyo - I change my dog for yours
Te compro eso por 15 euros - I am going to buy it for you for 15 euros
Para:
1. The final result or a purpose
Un vaso sirve para beber agua - A glass serves you to drink water
Vamos a Madid para aprender español y pasarlo muy bien. - Let's go to Spain to learn Spanish and to have great fun.
2. If there's something that is like a destiny for you
Mi amor es sólo para ti - My love is only for you.
¿Para quién es este regalo? - Who is the person who should receive this gift?
3. Opinion
Para mí www.italki.com es una página web muy interesante - I think www.italki.com is an interesting web page.
4. Direction
Vamos para la casa (Vamos a la casa) - Let's go home
5. Limited time
El debere es para viernes - The homework is for Friday
6. Consequences (however)
Para lo poco que comes, estás muy gordo. - You eat a little. However, you are very fat.
I hope this helps. If you have any further questions, feel free to ask me.