Here is the technique:
Improve one area in your artwork by putting all your focus and energy into it. Do this with each of your new artwork and you will see the results soon enough.
Find something on your artwork you want to work on and get some reference photos about that subject matter. Create quick studies and apply your newly got knowledge on your main artwork.
You will learn to draw and paint the subject matter by focusing on understanding it. You give this focus to it by spending time to draw (or paint) it.
If you can spend time to draw and paint one chossen subject matter with more care then each time you create a new artwork you will learn a lot and your knowledge will be more thorough.
Of course you can improve your artwork by other ways as well. I have some other tips on how to improve your art, you can read them in my blog here: