TITLE: How to write math well?
QUESTION [35 upvotes]: Let's learn about writing good mathematical texts. 
For some people it could be especially interesting to answer about writing texts on Math Overflow, though I personally feel like I've already mastered a certain level in writing online answers while being hopelessly behind the curve in writing papers. So,

What is your advice in writing good mathematical texts, online or offline?

REPLY [8 votes]: One useful strategy (after a few years' experience) might be to read your own papers from a few years before. Do you still understand your own proofs and explanations ( or would you, if you were reading them from "scratch" without your own prior knowledge)? If the answer is "no", or "with difficulty", you might ask yourself why, and what could be done to improve that in future writing.