(1) Letters written by you, signed by famous professor or professor closely related to the school you are applying;
(2)Letters written by you, corrected by those people as refered above;
(3)Letters written by you, signed by common faculty professors;
(4)letters written by you, corrected by those people as above;
(5)Letters written by famous people and you have no access to it;
(6)Letters written by common ones.
Choose you letters in the order above, and there are many sad experiences concerning the case against this basic rule: I was refused by 4 top schools just because I have used a very famous academy memeber's four reference letters. There are many other skills concerning the resume, etc. which I will cover next time. I guess as a conclusion, to make your reference letter useful, you must,
(1) Make totally good words in it, or make it more realistic, put sth. bad but not improtant at all;
(2)Choose relevant people to write it, either they do not know, or they know very well, Academy member in other fields will equals to common ones, or even worse because you can not get good words from them generally speaking.