For Gesture drawing
The key to gesture drawing is to use speed. Don't let your hand slow down. Let your arm be loose and quick. The goal here is to get an impression of what you are trying to draw. If you let your hand and arm slow down you may be tempted to try and start adding in detail to your drawing. Don't let yourself do this. As soon as you find yourself slowing down a little bit, get your speed back up.
When drawing shading the most important thing is to understand about that it's all about pressure - the pressure of the pencil, the pressure of your hand, all the gradual and subtle pressure in between. Once you understand this, you'll be well on your way to mastering this technique.