cridamour nailed it. regardless of where you go, the process of getting
LOR's is more or less the same. you find professors that know you well and that can speak well to you abilities. the earlier you get to know them, the better. what you don't want is a professor writing you a letter saying 'person X got an A in my class, but other than that, I don't have anything to say.'
besides, if it is any consulation, a very small percentage of students go to schools where they have the luxuries of having Mankiw as a prof....or even Don Campbell.
