Ah, this has been the source of quite a few snarky comments and PMs each time I post them. They either act like they know it all "oh it's just a so-and so-filter, I totally know how it's done but can you tell me anyway", or they'll ask for a tutorial and accuse me of withholding my "secrets" or writing terrible instructions when they can't emulate the results.
I'm not talking about you of course. It is hard to say how it's done without mentioning how a lot of people have asked me but failed one way or another. I will sound patronising at times, but please understand that it's necessary because it's impossible for me to know the reader's skill level with PS.
I used to post the tutorial I modified my methods from but it seems that people are just following it blindly.
Before you start:
You'll have to know the basics of colour adjustments because I did that a lot - I won't be able to provide the exact values of my adjustments because it was all done intuitively from experience. Besides, the adjustment slider only goes two ways: left and right. If it doesn't look nice one way, then go the other way.
Select a good quality source image because this effect was applied to all the images in my posters but only the ones with high-res, indoor lit portraits as a source image have the best result. I thought it turned out pretty well for this image, but it's a full body photo and there's no details in her eyes. People tend to react to portraits better because the results are more dramatic with this effect.
I usually work on images around 2000 to 3000px tall, so bear in mind that the filter settings may not appear the same on your image if it's just 500px.
After opening image:
Create slight S-curve with curves adjustments to bring out the contrast between the dark and light areas of the face. If it is a flat or outdoor image, I'll use the dodge and burn tool to create the areas of contrast manually (triangle across the nose bridge and cheekbones, etc), example here and here.
Vibrance and Saturation adjustment (+ Vibrance, - Saturation).
Use the shortcut CMD+J to duplicate the background layer twice, then with the uppermost layer selected add a High Pass effect from the Filters menu (less than 3 pixels radius, just enough to get an outline of the features).
Change the blending mode of this High Pass layer to Linear Light.
Merge the High Pass layer with the background duplicate below it, then Filter > Stylize > Diffuse > Mode: Anisotropic.
Filter > Sharpen > Smart Sharpen to bring out some of the details (100% with low pixel radius).
Filter > Blur > Surface Blur to remove the swirly paint strokes from large reflective areas of the face (cheeks, forehead, etc).
Image > Adjustments > HDR Toning to create a slightly overdone "Shitty HDR look".
Use curves adjustment to create a big S-curve on the untouched background layer. CMD+J once more and put it above the "painted" layer. Filter > Other > High Pass between 20 to 50 px. Set blending mode to "Overlay" and adjust opacity to between 20 to 50%. This is mainly for eyes and hair detail - I'll usually mask out the other areas with a big soft brush.
Change layer opacity of painted layer to between 50 to 80% so you'll end up with: High Pass layer (overlay 20-50%) > Painted layer (50-80%) > High contrast background layer
Use it as is, or merge layers and create a cutout with the pen tool with 2px feathering on the edges. At this point I'll usually add a final colour adjustment to tweak the reds and yellows.