I found Harper's to be helpful. The content seems to be careful and the writers really put together good-sounding pieces while paying attention to rules of style.
Somebody once told me to read counterpoints, like Cosmo and Esquire, or The Nation and The Wall Street Journal (Nobody hold me on the WSJ-- I can't seem to think of a solid right wing publication at the moment.) I've done this throughout undergrad, and its helped me improve my marks on philosophy and social science papers. Reading both sides makes you a more careful thinker.
There's a mid 20th Century justice (I wanna say Hugo Black, but I'm not sure) whose opinions are supposed to be very good for study--agian, i know its from a mid 20th century bench, just dont know who exactly.