Keep your pencils SHARP! When doing any kind of linework for a initial sketch I strongly recommend keeping the tips of your pencils sharp. Mechanical pencils can work, but they do not work for a really defined piece of art. A point of a pencil can cut into a edgeline without a muddled appearence like a mechanical or dull pencil can leave behind. I dont smudge, some artists do, but to get the texture that so many artists crave and for delicate work a sharpned pencil IS your friend.
You can always tell the distance to paper with a sharpened pencil unlike the unevenness in a dull or mechanical. Practice only filling in a small square without going back over your lines. Get the same shade throughout. This gives your hand a sense of applied pressure, as well as conditioning it like a reflex. Pretty soon you will be able to get a defined shadow, without even thinking about how you wrapped it around an object without smudging.