Practice

I'll start this post with a syllogism you will consider obvious to the point of laughability:

The more you write, the better you will get at writing.
Therefore, to improve at writing, you should practice writing as much as you can.

Once you've stopped shrieking with laughter at the obviousness of those two statements, tell me: how often are you writing these days?

If you're not writing every day, or at least close to every day, no one should bother to take you seriously. And you'll probably write little of significance.

Until you start practicing more.

"It is odd that we never question the feasibility of a football team practicing long hours for one game; yet in writing we rarely give ourselves the space for practice."
--Natalie Goldberg, Writing Down the Bones