I drew Elves and Lamias and Kitsunes with huge tits. And I suck. How do I improve? What is the secret?

Practice. And study anatomy

There's two quotes that stick out in my mind:

Yogi Berra said:
(When asked on how to get to Carnegie Hall.)

"Practice, practice, practice."

Chuck Jones said:
"Every artist has thousands of bad drawings in them, and the only way to get rid of them is to draw them out."

That's it. Studying anatomy and using references (but not tracing!) are part of that practice, but they mean nothing unless you put pencil to paper (in a metaphorical sense if not a literal one) and draw.

Chuck Jones said:
"You've got a million bad drawings in you; you better get started."