Add a layer effect to this layer. Inner shadow, change the color to a dark skin color like a deep orange red. Keep the inner shadow set to multiply, and decrease the distance to zero, and the size to 1 or 2. This makes it look like the hairs are shadowing the scalp, and you can paint on your actual layer to show the roundess of the scalp, because all the hair shadowing comes from the layer effects
This is a PIMM original technique. It just came to me while thinking about what makes most painted hair so fake. Real hair breaks, and the shorter pieces will tend to stick out. To simulate this, grab the three pixel brush or the one pixel, and change the opacity controls from pressure, to "fade" play with the number of steps, I use between 15-45 depending on the size of the texture. This will force the stroke to start at full opacity and fade to zero over that many steps. Now brush into the direction of the hair. This simulates the broken edes of the hair. I normally do this on a new layer, and then play with the opacity of the layer before flattening. Use both light and dark strokes, and it will help make the hair look more realistic
Create a new layer set to softlight mode. Using your soft hairbrush, and a dark and light color (pick them appropriate to your hair style color) go in and reinforce the depth of those clumps you made earlier. This will help the hair read from further away in the game. The dark bits will make the hair seem like it has more of a sheen to it.
Now for tricks on making your 3D hair better. The hair looks good as is, but its kind of flat and lifeless, lets fix that. This is assuming you only have 1bit alpha, but it will work just as well if you have 8bit alpha. In fact it will look better if you do
Here is my alpha mask for the hair. When you are painting in the alpha channel, hit "~" to get the color image overlayed on the alpha. Hit it again to go back to seeing the alpha channel only.
You can see the edge of the hair looks much nicer with the alpha map. Now comes the fun part, making the hair more volumetric.
Shift click these faces, and select "clone to element". Now convert your face selection to vertices, by ctrl clicking on the vertices element on the right control panel. (this is all max specific)
Now convert your face selection to vertices, by ctrl clicking on the vertices element on the right control panel. (this is all max specific) Now subtract the top three verts from the selection, so that you can move just the bottom six. this will make it look like the hair all stems from the same place. This makes the hair look thicker because as the model moves, you will see the inner plane move behind the outer plane. This parralax will make the hair seem much more voluminous than it is, and helps to kill the plane illusion
I did this again at the tip, with the bottom two planes. What can enhance the illusion even more, is to darken the under planes using vertex lighting if you have that available. Or you can flip the uvs so that you cannot see the mirroring of the texture from one layer down to the next. be sure to ofset them both in length, as well as slide them a bit to one side or the other, for a more random hair look.
I added one more flip outwards with a single plane. Here you can see what it will look like in a game with the wireframe turned off.
Create the initial volume quickly with the soft brush. It is very forgiving, and will give a rough hairlike look right away. Do not move past this stage until the hair reads as a volume from far away. Move on to picking out individual clumps next, so that the hair reads as fibers at any distance. Next use the sharper brushes to detail the hair a bit so that it looks nice from up close. Use soft light layers to enhance the sheen look, and deepen the volume whenever you need to. Use the smudge brush with these hairbrushes to blend. Add random tips if your resolution allows it. Use multiple planes to enhance the volume of the hair. Use alpha maps when possible to soften the edges of the hair. Use Lasso selections with a bit of feathering for highlights. Have fun, and explore new techniques!