How can I protect my pencil & charcoal drawings once complete?
If the pieces are likely to be handled and/or displayed then using fixative sprays is probably the best option. There are two purposely manufactured types:


Workable
Final


Workable Fixative

As the name suggests, this allows you to add additional layers to your work after the spray has been used. 

Workable Fixative is a thin solution and it sets up a new toothy (slightly rough) surface for more drawing. You can choose to spray the entire piece or isolated parts. To prevent other areas getting sprayed you can use paper, or frisket, to shield the areas that you don't want to spray. Once the fixative is dry you can continue working on the drawing.

The fact that it provides new tooth can be an advantage as heavily shaded/worked areas can become smooth making it harder to apply further layers of graphite/colour to the piece. So, you can use a workable fixative every few layers to keep a fresh tooth.

Final Fixative

This provides a more durable surface than a Workable Fixative, but this should only be applied once you are certain that you don't what to make any further adjustments.

However, a Final Fixative can cause the piece to darken so many artists do not apply the this layer and simply stop at a Workable Fixative.

Hairspray

Hair Spray has been mentioned as a cheaper alternative to fixative sprays. Below is a summarisation from Drawing for Dummies


  Hairspray does contain some of the materials of a Fixative but it only works for the short term, and ultimately damages the drawing - the hairspray yellows over time and ruins the drawing.


Do not use hairspray.

Applying Fixative


Use proper ventilation. Fixatives smells and is hazardous to your health.
Shake the can before spraying and test on a scrap piece of paper away from your drawing. The nozzle can clog &amp; will deposit 'lumps' on your drawing
Directions on the generally say to spray 20-25cm (8-10 inches) from drawing. However, I spray 30-40cm (12-15 inches) from drawing. Make sure it is on a flat surface and not drafty.
Lightly spray first coat horizontally. Let dry for 15 minutes. Make sure the layer is even and smooth.
Lightly spray second coat vertically. Let dry for 15 minutes. Make sure the layer is even and smooth.
Subsequent layers may be added in alternating directions if needed.


The sprayed areas of your piece will become darker once you have sprayed it and it won't lighten once the fixative has dried.