"How do I learn to write well in English (or any language)?”
To improve your writing in any language, including a second language, there are two general and simple rules:
1) Read a lot.
2) Write regularly.
What to write and how much?
We suggest setting aside between 15 and 30 minutes daily for writing. It can be anything, but must be in a style that is reasonable formal and professional (that is, lacking the colloquialisms, abbreviations, or emoticons one might use in highly informal writing in social media or emails to friends). It must contain continued thoughts that are or could be developed. It can be a journal or diary that you write daily or emails to friends. It can be about anything at all: a film you saw, how you feel about some person know or have met, or some other experience, or some issue you saw on Facebook or read about in the paper. It can be an informal comment about something you came across in your reading in your field, or possible ideas for an upcoming paper. The most important thing is to do this practice and do it regularly. Meaning more or less every day.
Stanley Fish in How to Write a Sentence suggests some exercises for teaching yourself to write great sentences. Work like that could be fit in your daily writing practice and nourished by your reading. You might also mark in the margin and/or write down sentences that you read that strike you as great. If they strike you, they might be. Great novelists are often known for that. The New Oxford Guide to Writing also has some style suggestions.
More of your time should be spent reading. The more the better.
What to read
You should probably read a combination of reading in your field, and more literary texts for pleasure.
Read the news in a good newspaper like the New York Times (which is written for a college-educated audience, as many newspapers are not) instead of listening to it on television. You can get a subscription to read it online on your computer or other device. Consider subscribing to some good magazines or reading them online. Or reading blog posts by writers in areas of interest to you, which can be on current films or popular music or anything else, just so long as it is well-written. Read novels in English, preferably with a focus on those that are not translated from another language but written in English. Good novelists are experts at writing well, and the styles of writing in them are quite varied. They will also differ widely in level of difficulty in terms of both length of sentences and frequency and number of words with which you may not already be familiar (enabling you to expand your vocabulary). The typical prose style in a scholarly or scientific paper is a bit different, and with a narrower range of variation, but exposure to good writing of any kind will tend to improve your writing and habituate you to thinking more clearly, effectively, and eloquently in English. (A language that, among other things, has by far the most extensive vocabulary among languages).
If you are a graduate student, professional scholar, or advanced undergraduate who is preparing for graduate school, learn what are the major journals in your field and those with articles of interest to you, and read from them regularly, selecting the articles you find most interesting. Read nonfiction books written by professors or other schools in their field, and suggested in either reviews, which are usually 1-3 pages, in scholarly journals in the relevant fields (where the better academic books usually are reviewed), or one of the better general book reviews in English (e.g., New York Review of Books, LRB (London Review of Books), Bookforum, TLS (Times Literary Supplement), LA Review of Books, New York Times Book Review -- all of which review books by scholars, which are usually published by university presses . Consider reading some of the better popular American magazines like New Yorker, Atlantic, or Harper's (liberal) or National Review, Commentary, or New Republic (conservative), or the British political magazine The Economist (conservative). If you consider yourself on the left, the Verso Books website has links to useful blog sites on its main page and blogs containing discussions by and about various authors on sites within "Books" under their names. Or read literary magazines like Granta, Paris Review, and any of the numerous others. If you like film, read Film Comment, Sight and Sound, or the online journal Senses of Cinema. If you like contemporary art, read Artforum.
Level of Reading Material
There is a rule in language studies of "N+1." If your current level in the language is N, you should read at level N+1. That means that on every page there are some new words, but not so many of them that it makes reading difficult. It's just like if you are learning a sport like tennis. Ideally, you want to practice with partners a little bit better than you, because then you will be more pushed to improve your own game.
If you read enough good writing, and also practice writing, you will over time become a good writer. You will pick up stylistic habits, and if you read enough of a variety of styles, you will develop a good stylistic repertoire, which is useful in enabling you to be in full control of your stylistic choices, using one kind of phrase or sentence for one purpose and another kind for another or simply variety.
More on suggested newspapers and magazines
Also, you should a good newspaper like the New York Times, which is the best and most influential newspaper in the United States and is written for upscale, college-educated readers. If you get your news from television or any online audio-visual source, stop now. There's too much personality in it and not enough of the kind of thinking that, unless you are a documentary filmmaker, is best done in and with writing. It is true that there are now more specialized news outlets that you can find on the Internet and that may better fit your politics or particular interests, but there are also blog sites that meet these needs in writing. The New York Times is liberal, its editorial page a bit less so, though the Op-Ed essays are typically provocative. The closest there is to a good conservative national newspaper is the Wall Street Journal, which is much more selective in its new coverage, though equally well-written. The Economist, a magazine, is also conservative and quite well-written. The best newspapers and magazines are a pleasure to read not only because they contain ideas that give food for thought, but also because they are written in a style that leads you to suspect that the journalists were all English lit majors, which of course most of them were. The three best literary book reviews in English are the London Review of Books (LRB) (which is left-wing and oriented towards people with an interest in contemporary scholarly work in the humanities), the New York Review of Books (left-liberal or center-left, its typically long essays are quite good and they write about politics as well as literature), and the Times Literary Supplement (British, conservative, short reviews). The LA Review of Books is left-of-center and good. The best all-purpose magazine in the English language is the New Yorker. It historically is the leading magazine for publishing short fiction, and also has excellent essays on literature, film, politics, and other things. It is liberal in a slightly left-leaning way. Other excellent magazines include the Atlantic, Harper's, Commentary (conservative), Artforum (a monthly New York-based magazine of contemporary art, it is left-leaning).
The advantages of reading literature are that literary writers are usually the best writers. There are some exceptions to this. In France, where philosophy is king, there are philosophers who are great writers: Sartre, Foucault, in some ways Derrida come to mind. And there are others who are just models of clarity in writing style (Rancière, Badiou are good examples). Usually though it is novelists, dramatists, poets and literary essayists (in English, Susan Sontag is a great literary essayist, though her style is literary and not academic even when she is writing about writers and thinkers; and there are others). The literary essay was invented in 16th century France by Montaigne; his term "essai" means attempt, and he used the format to explore ideas rather than present a linear demonstration, as is generally the case with scholarly essays today. You can find literary essays in the annual collections of "The Best American Essays for (Year)."
If you are learning the language, linguistic level is an issue. It is possible to construct a guide to major English language literary writers organized from easiest to most difficult; stay tuned to this site. Hemingway is easy. Shakespeare is very difficult. Melville, America's great writer, is not usually hard to read. Faulkner writes in long sentences and is harder. Nabokov and Pynchon are difficult. Salinger is easy. Among novelists: Austen, Dickens, George Eliot, Thomas Hardy, Joseph Conrad, Mark Twain, Virginia Woolf, Fitzgerald, and others are not difficult. The literary scholar Harold Bloom has a list of the principal books in the Western literary canon, divided by country and period (http://www.listology.com/marslike/list/harold-blooms-western-canon). You can sample or download online many classic literary works for free or very inexpensively on amazon.com and elsewhere.
Scholarly and scientific writing as stylistically between literature and philosophy
Writing well in university contexts is in a way a combination of writing well and making good arguments. You will best learn the first from novelists, for the most part, and the second from philosophers. Scientists and scholars at their best make arguments, but it is safe to say that philosophers have specialized in making arguments, and in a way they do so "purely" because, when they are doing philosophy at its purest in the traditional sense, they are not considered with empirical facts and observations or the features of artworks at all.