Keeping your drawing clean through completion is incredibly important. The last thing you want is to work hard on your drawing only to have your hand smear the work you created. Your hands contain oils that mix with the material and make the smearing that happens hard to erase and ugly. There are a few measures you can take to ensure that your drawing will remain clean, clear and crisp throughout it's completion.
1. Work on your drawing from top to bottom - Starting at the top of your paper and then working down will help keep your hands out of the way of the drawing action. This will go a long way to keeping your drawing clean.
2. Work on your drawing from left to right (if you are right-handed) - Starting on the left side if you are right-handed and from the right side if you are left-handed will keep your hands clear from the work.
3. Place a paper towel under your hand - By placing a paper towel under your hand, you will keep your hand from making contact with your finished drawing surface.
4. Erase any smears as you go - As soon as you see an issue with cleanliness, take care of it immediately by erasing it.
5. Use the right eraser for the medium - Certain erasers are right for certain mediums. Make sure you are using the right eraser. For more on the different types of erasers see this post-> Different Types of Erasers
These are techniques that I use to keep my drawings clean throughout completion. The following is a time lapse video of a recent commission. You can see how I use these techniques to keep my drawing clean in the video...