Matte finish mylar film is an artist's best friend! Sturdier, yet more trasnparent than tracing paper, it easily erases so a sheet can last for years. It's also fun to draw on with colored pencils! (See our Stones Kit)
And it's very useful as a "safety net" - place over your reference photo, trace an area (maybe a mouth you're having trouble with?) then place over your art. Since you can see through it, you'll see where you've gone astray on your drawing.