I'm not an expert but i would say that studying anatomy and the human figure (muscles) and going out of your confort zone (Drawing things that you find difficult/ it's not your art style) also help, even if you draw something that you seem that you wont use you can learn some stuff from drawing it.
Learn from the artist that you admire the most, look at their draws and try to figure out how did they do it. And of course do some research about them! If they have books or if they give advice to draw, sometimes little tips can be very helpful.
And of course, draw. As much as you can or as much as you have the time to do. You know what they say, practice make the master.
This is more personal but i found Jim Lee tutorials on youtube very useful, you should check them out. I see that you already do know how to draw persons so it wouldn't do bad.
Good luck and keep practicing!