The biggest part of writing is experience. You cannot become a better writer if you do not write. Even if you have written a terrible piece, you may still learn something that will help you improve on the next piece. Business writing is usually simple, there are templates available for a business letters, memos and reports. There is no need to have an artistic flair to your writing, it is about getting the point across to the reader in as few words as possible. This may sound like a massive buildup that might have you concerned about anything that you write in the future but there is hope.
An article by Jeff Haden on Inc.com gives 10 tips for business writing. Getting the point across, using the standards that already exist and focusing on who you are writing to are some of his tips.
You can read his entire article here.