There are three rules for writing the novel. Unfortunately, no one knows what they are. (W. Somerset Maugham)

Everyone has to develop their own writing style, based on their own strengths and weaknesses, on the subject matter, on the target audience, and sometimes on the target medium.  As such, it is virtually impossible to prescribe rigid rules for writing that encompass all conceivable situations and styles.

Nevertheless, I do have some general advice on these topics:

I should point out, of course, that my own writing style is not perfect, and I myself don’t always adhere to the above rules, often to my own detriment. If some of these suggestions seem too unsuitable for your particular paper, use common sense.

Dual to the art of writing a paper well, is the art of reading a paper well.  Here is some commentary of mine on this topic:

Some further advice on mathematical exposition: