Have you ever tried timed drawing? This greatly improves your overall drawing skills when done daily. Do this directly with a pen to avoid feeble, uncertain lines sure to come in with a pencil because you know at the back of your mind that the eraser will always be at hand. Here my exploratory quick sketches with a fountain pen in my sketchbook cleaned up in photoshop, of animals, 30 seconds to 1min each :
Watercolor Journal 