Excellent advice from various pros:
From William Safire (author of the New York Times Magazine column “On Language”)
Tips in which he cleverly commits the very sins he warns about:
1. Remember to never split an infinitive.
2. The passive voice should never be used.
3. Do not put statements in the negative form.
4. Verbs have to agree with their subjects.
5. Proofread carefully to see if you words out.
6. If you reread your work, you can find on rereading a great deal of repetition can be by rereading and editing.
7. A writer must not shift your point of view.
8. And don’t start a sentence with a conjunction. (Remember, too, a preposition is a terrible word to end a sentence with.)
9. Don’t overuse exclamation marks!! (I never use any!)
10. Place pronouns as close as possible, especially in long sentences, as of 10 or more words, to their antecedents.
11. Writing carefully, dangling participles must be avoided.
12. If any word is improper at the end of a sentence, a linking verb is.
13. Take the bull by the hand and avoid mixing metaphors.
14. Avoid trendy locutions that sound flaky.
15. Everyone should be careful to use a singular pronoun with singular nouns in their writing.
16. Always pick on the correct idiom.
17. The adverb always follows the verb.
18. Last but not least, avoid clichés like the plague; seek viable alternatives.
A few more nuggets:
“Never use a long word where a short one will do.” —George Orwell
“Don’t sit down in the middle of the woods. If you’re lost in the plot or blocked, retrace your steps to where you went wrong. Then take the other road.” —Margaret Atwood
“Write first and always. Painting, music, friends, cinema, all these come afterwards.” —Henry Miller
“Never use a verb other than said to carry dialog.“ —Elmore Leonard
And my favorite: “Write.” —Neil Gaiman