How can I reduce harsh facial shadows in post-processing?

I don't know of any "easy" way to fix an image like this. I don't know of any way that could be done purely in Lightroom either, although you could do some tweaking with the shadows and highlights sliders before bringing it into Photoshop for the full correction. It is possible to fix it, however, using a combination of dodge/burn for selective shadow and highlight recovery, and a multiscale soft contrast technique.

First off, here is my version of your image (just processed from the JPEG you shared, and done rather quickly, so certainly not ideal):

You should notice a few things. First, the background is a bit darker, making the foreground and the model pop a bit more. The color in the car has been enhanced a little. And, the shading on the model's face should be improved. Here is a before and after (before left, after right):

The changes are subtle, but effective, I think (and they could be much more effective if you have the original RAW data to work with, instead of a JPEG). Note the falloff into the shadows around her neck as it goes back into the darkest shadows. Note the lighting on her face. Also note the tonality of her dress.

This was accomplished in a series of dodge, burn, and soft contrast adjustments in layers, many with layer masks. I don't have any technique site to direct you to, I simply processed to achieve my goals. Here is the procedure I followed.

1. Dodge - Selectively Lighten Shadows

   • Eyes and neck

2. Burn - Selectively Darken Highlights

   • It is important to note that I darkened the highlights a lot more here than they appear in the final image. This is due to the nature of the soft contrast technique, which tends to darken shadows and brighten highlights, while softening the transition between them. Darkening her dress and face was critical to supporting the soft contrast.

3. Global soft contrast layer

   • Copy Merged, Paste, (Paste Again, rename as "Soft Contrast - Base", hide), Select First Paste of Copy Merged again, Gaussian Blur 20, Blend: Overlay, Fill Opacity: 20%

4. Soft Contrast - Small

   • Copy "Soft Contrast - Base", Rename "Soft Contrast - Small", Gaussian Blur 3, Blend: Overlay, Fill Opacity: 35%, Layer mask: Black, paint in model

5. Soft Contrast - Medium

   • Copy "Soft Contrast - Base", Rename "Soft Contrast - Medium", Gaussian Blur 8, Blend: Overlay, Fill Opacity: 20%, Layer mask: Copy from #4

6. Soft Contrast - Large

   • Copy "Soft Contrast - Base", Rename "Soft Contrast - Large", Gaussian Blur 12, Blend: Overlay, Fill Opacity: 10%, Layer mask: Copy from #4

7. Curves
   • Applied several curves adjustment layers to tweak exposure levels
   • Two to darken highlights, lighten shadows on model, Layer mask: Copy from #4
   • One to tweak tones for everything else, Layer mask: Copy from #4 and invert

8. Sharpening Overlay
   • Copy merged, paste, Smart Sharpen: Amount 50%, Radius 0.3px, Reduce Noise 15%; Repeat two more times, Fill opacity: 70%

(See original image for final result.)

Soft contrast is a great way to smooth out tonal transitions. It won't correct hard shadow edges, but it will help mitigate the tonal grade across them. The application of multiscale soft contrast techniques like this will usually increase contrast in a dramatic way, so it is important that you mitigate that by burning bright areas in a layer before you add the soft contrast layers. This will pre-darken those areas, mitigating the brightening effect. I applied a light amount of burning to the model's dress and face. Too much and compression artifacts showed up, and the highlights were heavily clipped in a few places.

You will also often get deeper shadows, however the falloff into those shadows will usually be quite superior. You can mitigate this with some dodging in those areas before you Copy-Merge the base soft contrast layer (that is simply so you have a common layer to copy from for each soft contrast layer). I did light dodging in this case, as due to the fact that the original data was a JPEG, too much and it started to fall apart.

With original RAW data loaded through ACR or converted to TIFF, you should have MUCH more freedom to dodge and burn. You could also apply some highlight and shadow sliders in Lightroom or ACR before performing the steps of my procedure above to mitigate both ahead of time (particularly the highlights, shadows are probably better controlled with dodging as you can do it selectively and locally, thus avoiding any loss in global contrast.) In the final image here, the highlights are still fairly hot, but the model's dress has a bit more detail. Her face, I think, is a lot more pleasing with the softened highlights and smoother shadow falloff around her neck.