How to practice to improve your skill. This will work with any task or job you have to complete.
I found this short but useful video blogger who gives great tips and shares a lot of his how-to-do art.
He speaks off 3 types of practice that will help you improve.
1- Innate practice - chose a project and work on it everyday for a year or two, in the end you will see how much you have improved. Notfrombrazil project started with no style or color for each cartoon, a year latter I established a style and remained with the same format. It helped me to improve my skills a lot.
2- Inspirational Practice - Keep a journal, paper and pencils always with you, so when a music, image, movie, show, walk around the museum inspired you take notes or produce drafts. This is the most interesting kind of practice because it deals closer to motivation. Do what you like. Nonetheless, it will burn out quickly.
3- Developmental Practice - Follow tutorials, step-by steps, learn from art books, practice your weaknesses. He recommends several useful exercises. ( life drawing, simplification of shapes into silhouettes or geometrical shapes, and more)
The best is to balance between all three and use them everyday.