- Avoid run-on sentences that are hard to read.
- No sentence fragments.
- It behooves us to avoid archaisms.
- Also, avoid awkward or affected alliteration.
- Don't use no double negatives.
- If I've told you once, I've told you a thousand times, "Resist hyperbole."
- Avoid commas, that are not necessary.
- Verbs has to agree with their subjects.
- Avoid trendy locutions that sound flaky.
- Writing carefully, dangling participles should not be used.
- Kill all exclamation points!!!
- Never use a long word when a diminutive one will do.
- Proofread carefully to see if you any words out.
- Take the bull by the hand, and don't mix metaphors.
- Don't verb nouns.
- Never, ever use repetitive redundancies.
- Last but not least, avoid cliches like the plague.
The following is taken from "How to Write Good" by William Safire. It is a useful and humorous list of what you should avoid in order to improve your writing.