The Economist published their writing style guide online which they hand out to every journalist there. I find that their section for clarity of writing is very useful on its own.

Clarity of writing usually follows clarity of thought. So think what you want to say, then say it as simply as possible.

Here are George Orwell's six elementary rules from "Politics and the English Language", 1946:

1. Never use a metaphor, simile or other figure of speech which you are used to seeing in print.
2. Never use a long word where a short one will do.
3. If it is possible to cut out a word, always cut it out.
4. Never use the passive where you can use the active.
5. Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent.
6. Break any of these rules sooner than say anything outright barbarous.

Source:
The Economist Research Tools Style Guide