Draw longer, faster lines, be confident and don't scribble. Make sure you work on a sufficiently large canvas at 300dpi+ to prevent it from looking pixelated.
But really, most of it is just plain practice and working on your draftsmanship. You need to put in the mileage to get your muscles to do what you want to do, just like in sports. And, well, that takes time, unfortunately.
If you want some more specific advice it'd help if you could upload some screenshots of your work!