nice drawing! bit of advice: i noticed you take your time and sketch slowly as if you're trying to get it right the first time - try "loosening up" your tecnique a bit :) get real sketchy during your "blocking in" stage or when you're just trying to get a basic image down. lower the opacity (clarity/solidness) of your brush - this will make it so each time you go over a previously drawn line it will become darker. this is a great way to sketch digitally. and using a thicker brush than the pin-point one you used here should help. and don't worry about making mistakes - a drawing is like a marble statue; you start with only a big rock and gradually work your way to the fine detail.