In the same domain as The Chicago Manual of Style (users working "with magazines, newsletters, corporate reports, proposals, electronic publications, Web sites, and other nonbook or nonprint documents"), you also have:
The Economist Style Guide
Clarity of writing usually follows clarity of thought. So think what you want to say, then say it as simply as possible.
Keep in mind George Orwell's six elementary rules ("Politics and the English Language", 1946):
- Never use a metaphor, simile or other figure of speech which you are used to seeing in print.
- Never use a long word where a short one will do.
- If it is possible to cut out a word, always cut it out.
- Never use the passive where you can use the active.
- Never use a foreign phrase, a scientific word or a jargon word if you can think of an everyday English equivalent.
- Break any of these rules sooner than say anything outright barbarous.