The best way to improve at anything is through discipline and repetition. If you are serious about becoming a better writer, I recommend you give yourself eight weeks, five days a week, five minutes at the end of each day and between thirty minutes and three hours at the start of the next. Decide in advance how many hours (between 20 and 120) you will devote to this effort over those eight weeks. Now, just follow the rules.
I have devised an eight-week challenge to help you focus. I will presume you are already motivated to improve your writing and have found my description of Writing Process Reengineering intriguing enough to give it a shot. In this document, therefore, I offer very little in the way of inspiration or encouragement. You will find some instructions, reporting tables, and a copy of the rules. The rest is up to you. But you are of course welcome to contact me at any time to ask for advice.
*I recommend the Word version since it will let you fill in the tables, modifying them as you choose.