How can I avoid smudging my pencil drawings?
You can use an artist's bridge as well. It's basically a long metal ruler with either rubber feet at either end or a thin foam on the bottom. You can easily make your own out of a sturdy ruler as well. These are used by illustrators but could be useful to the fine artist as well. Here are some examples:

 

The bridge is great for horizontal work but for easel work you can take a page from Salvador Dali's playbook and use a mahl stick. A stick with a ball of leather or rubber is a mahl stick. For the right-hander, the stick is held in the left, the ball is placed at upper right, either on the work surface or edge of easel. Some artists use a leather thong or nail to secure. You now have a movable bridge to keep your hand off the work surface. I use a yardstick with a ball of rags and tape at one end. You can easily make different lengths based on the scale you work at. Side note: see John Cusack demonstrate the use of a mini mahl stick in "Being John Malkovich". Try a wooden dowel with a tennis ball or solid rubber ball at the end.

Also, there is the issue of oils on the hand. To battle this use thin cotton gloves to handle paper. You used to be able to get these at photo supply stores. Cut all fingers off but the pinky for drawing hand. Again, this really just depends on how much you care about preservation of the white of the paper. For illustrators this is huge.

I add the following for thoroughness and the OCD tendencies in some of us. Use frisket film to protect areas you want crisp and white. I've used this for charcoal and the effect is quite profound. Sharp crisp edges, perfect, immaculate whites contrasted with loose charcoal painting and drawing is almost magical. Tedious? yes ....worth it? I leave that as an exercise for the reader.

For simplicity and ease of use, the extra piece of paper suggestions are great. Just adding my 2 cents for completeness. May I just add that tracing paper is great for this due to its lack of tooth and slippery surface.