1: Draw a big box – no fill, no stroke – centered on the centerline of your skull sketch.
2: Draw a few other quick shapes just so you can see the magic happen.
3: Target the layer in the layers palette (click on the little circle to the right of the layer's name).
4: Effect->Distort and Transform->Transform. 1 copy, turn on reflect X, leave everything else as-is.

Voila: everything drawn in that layer is automatically reflected. As you draw new shapes, they too will be reflected!

5: Select that box you drew in step 1 and lock it so it doesn't get moved by accident.

Also, most of your little feathery lines are just begging to be done as an art brush. Draw one straight line with some weight variance to it, drag that to the brush palette and select 'art brush'. If you draw it in pure black (C0M0Y0K100 or R0G0B0) and choose 'tints' as the colorization method then the brush will be whatever color you choose as a stroke when you use it; very handy for scratchboard-type stuff.

thanks