Writing is difficult. This is true for professors and students alike. The best way to improve your writing is to read good writing -- it is contagious. For a quicker fix, try one of the links below.
General writing:
Politics and the English Language (pdf) , George Orwell, 1946.
Why Academics Stink at Writing (pdf), Steven Pinker, 2014.
Writing to Learn, William Zinsser, 1993.
On Writing Well, William Zinsser, 1990.
The Craft of Writing Effectively (video), Larry McEnerney, 2014. Great video on academic writing.
Writing economics:
Economical Writing (pdf), Dierdre McCloskey, 1985. The article was expanded into a book.
The Young Person's Guide to Writing Economic Theory (pdf), William Thomson, 1999. It has also been expanded into a book.
How to Build an Economic Model in Your Spare Time (pdf), Hal Varian, 2009. A guide to economic research with some writing tips.
Slides on writing economics papers: