Go to Filter > Liquify... (Ctrl+Shift+X) and wait a moment for it to load (it always takes a bit). First thing you'll want to do is check the Show Backdrop option, Use: All Layers, and Mode: Behind.
The image below has all of the important areas circled. You'll just use the default tool, but you can feel free to experiment with the others. The great thing about Liquify is that it lets you nudge around things without blurring the image. The brush size is really sensitive. Change the brush size as needed and move stuff around.
When you're done in Liquify, just click OK and it'll update your main work image in Photoshop. Save and check stuff in 3dsmax to see if anything needs nudging in different locations. I did this and I saw that the ears were still a little messed up, and the hair line needed some work.
So I went back to Photoshop, did Liquify again, and adjusted the shape and alignment of the hairline so that it'd display better on the actual model.
Okay, now its time to clean up some of these overlaps. Often times, using the Eraser with a fuzzy brush, and going over the edges of each layer are enough to blend things together quite nicely. If you need to, you can use the Clone Stamp Tool as well.
And another save and check in 3dsmax. Everything seems lined up good. The forehead still needs some work, but I can clean that up later when I start doing manual touch-ups. I decided that the eye layer could probably be stretched a little bit in width (eyes looked a little small anyways) and it'd fill that annoying gap without having to mess with the clone stamp tool.
At this point I copied the neck from another reference image, and the back of the head from yet another. Use Liquify where needed to nudge things into place, and erase to smooth the transition between layers.
The picture that I had as reference for the back of the head was actually taken from a different person and the skin tone didn't match. It's common that you'll have to piece together the facial texture from several sources and getting the skin from each piece to match can be a real pain. But there's one quick trick I've found that works most of the time.
Ok, so you've pasted the piece onto a new layer. Go to Image > Adjust > Match Color and a window will come up. In the bottom portion of the window is Source and Layer. From the Source drop-down choose the image you're working on now. Now from Layer pick a layer that you want to match.
Since the layer I added had hair, and skin, I want to match it to another layer that has both hair and skin (the one I sampled from was the forehead/top hairline). This way it will match better.
If you need to, adjust the Luminance, Fade, etc. Match color doesn't always work, but it usually gets pretty close.
So just continue grabbing pieces of photo ref, use match color and Liquify when needed, and check 3dsmax frequently to make sure things are matching up.
This is the base texture I ended up with. At this point I *could* be done. But I'm not. Now I'm going to go in and manually paint in some details, sharpen some areas, and apply general touch-ups all over. Since this is a low-res texture, and the character will usually be seen from a distance sharpening some features usually gets a better result.
At this point, I selected all of my face layers and merged them together. I created a new layer on top of it (but still below the UV ref layer) and selected the Paintbrush tool. In the options set the Opacity to around 50% and choose one of the solid round brushes.
Holding down the Alt key will switch your tool to the eyedropper so you can pick colors quickly and easily. While painting you will frequently need to pick colors from the image so just keep your finger on the Alt key for easy switching.
The texture as it currently is freaky on the forehead when I actually check it in 3dsmax, so the first thing that I touched up was the forehead.
Hold down Alt and pick up some skin tone. Now, just paint over the problem areas with larger brushes. Since the brush is set to 50% opacity, you can get in-between colors by doing a singe brush pass over an area, and then using Alt to pick the new color. Do this over and over and make layers.
For smaller details, switch to a smaller brush size. I defined the hair line by just picking a black-ish color from the hair and painting in better hair roots.
After the forehead, I turned my attention to the eyes. Using the eyedropper I picked the 'white' of the eye. But then I went into the color picker and went brighter (still not actual 'white' but a lighter color then the one there) and painted in the whites of the eyes better.
I picked lighter skin tones and increased the contrast around the eyes and the wrinkles, and I sharpened up the eye brows.
I checked the progress in 3dsmax and deiced I still needed more definition to the forehead. Went back in and did some more touch-ups to the hair line and checked it in max.
My plan is to make some alpha poly hair once I'm down with all the textures, so this is just the under hair, but it still needs to look good on the edges and hairlines.
As another example, just compare the eyes of this version with the texture before any manual touch-ups (just above). The eyes stand out much more now. So, even small touch-ups can make a big difference.