If you're interested in learning how to write clearly, check out "The Art of Plain Talk". [1]

It's dated – from the 1950's I think – but shows how to express ideas with clear, direct language. A lot of it has to do with using verbs as much as possible and cutting down on adjectives.

Similarly, Hemingway has always inspired me.

Reading, in general, is very helpful. Read widely, on many subjects. Find authors whose 'voice' you like, and emulate it in your own writing. Stretch yourself with publications like The New Yorker, and Harper's.

1: http://www.amazon.com/Art-Plain-Talk-rudolf-flesch/dp/002046...