1. Duplicate the background layer by holding down the left mouse button
and dragging it down to the little icon left to the garbage can icon.
Do this often when enhancing hair, Alt+Ctrl+Z is limited, this will help you if you're experimenting.

2. If your screenshot is dark, I suggest tuning up the Curves. Image>Adjustments>Curves or Ctrl+M.
Adjust as needed.

3. This is where it gets difficult, it'll require patience.
If the RSI you're enhancing has hair lines on face like this one, you'll need to follow this step carefully.
If you're unfamiliar with the Clone Stamp Tool, follow this tutorial first: Click here.

Select the Clone Stamp Tool (S) Opacity: 100, Master Diameter (right click): 4-10, Hardness: 90-100,
carefully clone away the ends of the hair lines and try to get a more natural shape of the hair line.
This helps to get a more nautral look to the hair.
I'm afraid I can't explain it better, but this step will take some time to learn.

4. Now it's time to make some hair lines. This works best if you have a Wacom Tablet, but you can do it with the mouse.
Select the Burn tool (O), Range: Highlights, Exposure: 80-95, Master Diameter: 1-2, Hardness: 75-90.
Make hair lines everywhere on the hair, just make sure they're going the right way, or else it'll look like your RSI is having a real bad hair day.

5. More hair lines, but this time with the Dodge tool.
Just repeat step 4 but with the Dodge tool instead. However with the changes: Range: Midtones, Exposure: 40-60.

6. Select the Smudge tool (R), Mode: Normal, Strength: 90-95, Master Diameter: 1-2, Hardness: 90-95.
Use this on the hair, following the hair lines in order to make it look more natural.
Also make sure to smudge out a few lines of hair overlapping the others.

7. More smuding, but this time at the edges of the hair.
Mode: Normal, Strength: 95-100, Master Diameter: 1-2, Hardness: 90-100.
Smudge out hair lines at the edges of the hair. Be sure to create long hair lines this way.

You're done!
If you want you can edit the hair further to your liking, following the steps above.

I've played a bit with the light and shadow here in the final picture for more realism using the Dodge and Burn tool.

Update:

If you want to go even further you can use the Dodge and Burn tool with an Opacity of around 30-50 and a Master Diameter of around 10-30, depending on the resolution of the screenshot and how close to the actual face the screenshot is. You can also go further with the Smudge tool to create a more realistic feel to the hair, like so:

If you have any questions or find any errors please feel free to let me know.

I hope this helps.