1. Know the Rules
This doesn’t mean to simply remember what you learned — or what you think you learned — five or fifty years ago. Careful writers continuously educate and re-educate themselves about grammar, syntax, usage, and style. Confront your prejudices, and check your recall and understanding of the basics. Most important, don’t believe everything you think.

2. Be Open to New and Unusual Usage
Language changes, and writers must change with it. This doesn’t mean that you should abandon your high standards and accept colloquial language; some contexts simply do not allow for a relaxation of the rules. But most forms of writing are flexible, and you should be, too. Adapt the language to the content, but consider also adapting the content to the language.

3. Verify
When in doubt, look it up. When not in doubt, look it up. Don’t be content with spell-checking programs; check not only definitions of words, phrases and expressions but also their connotations. When discussing a person, place, or thing, don’t simply double-check the spelling and treatment of the term; reacquaint yourself with the person, place, or thing to confirm or correct your impression that the reference is appropriate for the content. (And check your facts.)

CREDIT: Daily Writing Tips