Well as simple as it sounds, you just have to draw as much as you can. But not aimlessly, if you're drawing specifically to improve then you should focus on the things that you struggle with the most. Working on you're weaker points will definitely urge progression.
Make sure you draw from life a lot as well. On the bus, at a life class, at home or anywhere really. You'll build up a good visual library of information at the back of your mind that is called upon when you draw from your imagination. Most professional and experienced artists will emphasize the importance of drawing from real life, and not just photographs.