What is a good practice for improving business writing skills?

Before you put pen to paper or hands to keyboard, consider what you want to say. Make your point right up front. Many people find that the writing style and structure they developed in school doesn’t work as well in the business world. One of the great diseases of business writing is postponing the message to the middle part of the writing.Avoid jargons and read what you write.

It's very time-consuming for me to write all my recommendations here.  Just check this site out and apply the tips:

5 Tips on Business Writing Skills 

Or, you may even search it in the internet.  There are a lot more articles regarding the subject.