The field of writing is hard-pressed on one side by the terror of grammatical incorrectness (epitomized by those whose abusive third-grade English teachers told them that splitting an infinitive was a capital crime), and on the other by the idea that good writing is easy (epitomized by would-be authors who torment publishing house editors with "sure bestseller" drafts that might well have been written by someone who flunked third grade). Both extremes have this in common: they result in writing that's deadly boring to read. And deadly to its intended purpose.
If you want your business writing to convince others to use your services (and to otherwise make a good impression for you), practice walking the middle line of readability.
To avoid the trap of ultra-correctness: Write the first draft the way you'd give the same information out loud. If you were giving a verbal presentation, you wouldn't say "you would not" or use an excess of fancy-but-vague words like "facility" and "ordinances." Pretending you're talking, rather than writing, to someone keeps your language sounding natural. (If you're seriously uptight about writing the way you speak, try dictating to a secretary or recorder at first.)
To avoid the trap of ultra-informality: Don't consider the writing finished with completion of the first, written-as-spoken draft. This is where most overly optimistic novelists and bloggers fail: they spill out their words stream-of-consciousness-style and expect the deeper meaning to be as obvious to others as to themselves. Give your work a few hours or days to rest, then review it word by word, considering what might be difficult to follow, what's redundant, and what jumps too abruptly from one topic to another. The close relationship between the written and spoken word applies here too, so reading a piece out loud can help. Getting someone else to read (or listen to) it is even better.
Remember: natural language first, then the polishing to the small degree of formality that distinguishes the written word from the spoken.
What are your favorite hints for avoiding one or both of the above extremes? Please comment.