The standard advice I give to people young and old who want to be authors is to read as much as you can and write as much as you can. It sounds trite, but really, it is the most important way to improve your skills.
What amazes (and saddens) me is how seldom I follow my own advice. School visits, the business side of publishing, day-to-day chores swallow my time, and weeks (or months) go by without me having written anything new (other than my daily journal entries). How often do I need to remind myself to write, write, write? And read, read, read? Obviously, I have to remind myself of this a lot.