The line work is good, but you should work more on shadows. And draw real things if you really want to get good.
I drew kroki for the first time a month ago, and its amasing how fast you learn new stuff when drawing a model. I really can't stress this enough: draw real stuff, preferably humans. You will immediately see what you are good at and what you arent good at, so you can work on what you don't handle well instead of just drawing wht you DO know. Draw someone at a caf?, or get a friend do model for you. ( No, it does not have to be nude... but it helps ).
Here is some of my doodles ( coal chalk on standard paper ):
