Hi fullbite, sorry for taking so long to respond.
I really need to make some time and write a full-on por/para tutorial, this is something that is constantly coming up and is something that plagues all of us learners of Spanish.
Concerning por usted and para usted, they're both correct, but mean different things.
Lo haré por usted. I will do it for you (doing something in their place)
Lo haré para usted. I will do it for you (they are the beneficiary of your doing it.)
Here´s a good writeup on por and para.
http://www.studyspanish.com/lessons/porpara.htm If you have any questions that I can answer more specifically, just ask.