Writing style is dictated more by corporate philosophy than by what is right and what is wrong. The Associated Press maintains an entire book so the writers who work for AP will know what style to use.
As time goes on, the rules become more and more numerous. there wasn't even a standard for spelling until the latter part of the nineteenth century when Noah Webster decided we needed a dictionary. Personally I think AP's book is too much in its entirety, but here are a few things to keep in mind when you write :
1- Use the Shortest possible set of words
Flowery adjectives and adverbs are out of place in business writing. Always use the shortest possible set of words to communicate the point.
2- Use the shortest word when you have a choice
Use "use" instead of "utilize" whenever you can. How often do you utilize the bathroom? Always use the shortest word when you have a choice
3- Always spell words correctly
Use spell check and use your dictionary. Proofread your work for incorrectly used words that are spelled correctly. You will be fired if you misspell the brand name or the name of your client.
4- Use good grammar.
Remember : if you make mistakes in spelling or grammar the reader will assume that you are uneducated and that there will be no reason to believe your point of view. The only other interpretation the reader may assume is that you do not care enough about this communication to check it.
5- Tell the most important thing first.
Don't make the reader wade through a lot of minutiae to find the good stuff. Put it up front.
6- Use topic sentences
Always make the first sentence of a paragraph the topic sentence. In English Composition class you may have learned that you should vary the topic sentence to add interest to your writing. This thinking does not apply in business writing.
7- Never start a paragraph with a dependent clause.
The reader will get your point faster if you put that phrase later in the paragraph.
8- Do not start sentences with prepositional phrases.
Again, the reader will get your point faster. However this is a harder rule to follow. In less formal writing, prepositional phrases at the beginning of a sentence seem to make the sentence friendlier. Still, in business writing, and especially in formal planning document, do not start sentences with prepositional phrases.
9- "The purpose is..."
When you start a memorandum or letter, you should tell the purpose for that written communication in the first sentence. It is common to write "the purpose for this letter is...". I quite often see people start all written communication with the word "this" :
"This provides ..." , "This responds to your request for..." or "This request..." are all example of this usage. "This" forces you to write the purpose of the memorandum or letter in that first sentence.
10- Never end a sentence with a preposition
Winston Churchill once said : "This is something up with which I shall not put." There is no preposition at the end of that sentence.
11- Use tabs on your computer.
12- Never start a sentence with an Arabic Number, write is out
13- Be specific.
If you want the reader to believe what you are writing, do not simply write "research indicates that..." . This kind of statement often creates more questions than it answers. What research ? When was it conducted ? Under what circumstances ?
14- Write as short as you can to communicate the point.
We all have more to read than we have time to read. Give your reader a break.
15- Avoid Superlatives.
In business documents, overstatement and "hype" are treated with suspicion. What may be appropriate for the advertising may not be appropriate for the advertising plan.