There are a lot of great rules to follow in this link about ways to improve your writing. It's broken down into 4 sections - General, Technical, Life, and The Method. Personally, I like the following rules the best:
Write like you speak - which is advice I've always heard but have trouble putting into play.
Punctuate. Few things annoy me worse than bad grammar and punctuation. It limits clarity and confuses.
And READ. That's the easiest part for me.
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