Work on projects.
Here's the explanation:
You can spend a lot of time sketching from nature and drawings nudes. You can draw a thousand little heads on notebook paper. You can work on your shading and line weight, etc.
Unfortunately, however, good draftsmanship is trumped by good design sense.
You're not going to learn design by sketching and making incomplete drawings. You learn design by finishing.
This is why working on projects is so important.
You can make up your own if you like. Maybe you've had this idea for a graphic novel kicking around in your head. Draw it! Are you in a band? Make stickers, fliers, and CD covers. Make a coffee cup painting for a local coffee shop...free of charge!
Having a project to work on teaches you how to solve relevant problems. It builds the skills you need to be successful in the marketplace. You learn how to make compelling work. You learn how to use space effectively. You learn how to make your drawings serve your purpose.
Don't just doodle. Work on projects.