Touch-ups on the nose and lips can make a BIG difference in the final look so they're important.
Usually the first thing I do is fix the nostrils (they usually aren't shaped correctly to appear in the right location on the actual model. Just make adjustments, save, check, and repeat until they look correct in max).
I made the base/underside of the nose darker, increased the contrast between the bridge of the nose, and the shadow on the sides of it. I adjusted the highlights on the outer nostrils, and drew in the crease where the outer nostril meets the cheek.
For the lips, I find it's best to make the top-lip almost solid dark. I drew in highlights along the top of the lip, and the highlights on the lower-lip. I also darkened up and created a more solid border below the bottom lip.
I find that I often get better results on skin if I use a small brush (usually the solid round 3) and 'scribble' in some areas. You don't want a really smooth look on the skin because it'll look fake.
Again - save often and check your progress in 3dsmax. Already the face is looking much more defined then it was before the touch-ups with minimal work.
At this point I just finished up the rest of the head. I did a lot of touch-ups on the ears, since they weren't lining up perfectly with the model, and defined the hair better. I also went in with a lighter color of the hair and added highlights.
Since the plan is to make poly hair later, all that I really care about is if the hair edges look correct, but it's still good to get it all looking as good as possible.
The last thing I do to any texture for low-res real-time is apply sharpen to it. Flatten the image and go Filter > Sharpen > Sharpen. Then go to File Save As...and save your texture as a .tga or whatever format you want to use for the final texture.
When you go back into max, edit the material for the head (click on the box next to diffuse and change it from the PSD to the new flattened image you just saved).
Comparing the end result now with the way the head looked before I started any manual touch-ups really makes it obvious how big a difference it can make.
Onto the Clothing
I do clothing differently then I do faces. I lay down a base texture and it's usually extremely plane and flat. I'm not looking to get any shades or wrinkles in the base texture. Just the actual 'texture' of the fabric, and maybe things like seams, pockets, zippers, etc.
Once my base is down, I go in and paint in the wrinkles using multiply and screen layers, which I'll cover later.
Okay, so I'm moving onto the pants next because I know exactly what I want for them (jeans). Good reference images are very important to make jeans actually look like jean. Getting that "jeans" look is not hand-painted easily. I had these images that I used to grab my textures from. Again - they came from http://www.3d.sk! Ha-ha - I'm a walking advertisement for this site, I swear... But this just goes to show that it's worth subscriptions. Every texture I've used on this guy was from that site, so I highly recommend it.
Click to Enlarge
The image may be of a jean skirt but it actually made it even more perfect since it has a lot more surface to grab texture from.
Open the UV layout texture for the pants, set it up just like we did with the head so that your UVs are on a separate layer with a blank background and save it as a .psd file. In 3dsmax open the material editor (m) and setup one of the blank material slots so that it's pointing to the PSD file you just saved and apply that material to the legs.
Back in Photoshop, copy portions of the jeans and edit it so it fills the whole background of the image. After I'd managed to get a solid backdrop of 'jeans' cloth, I started copying in seams and other details.
Remember, if you are having trouble lining anything up with the edges/seams of your UVs, that you can use Liquify (Filter > Liquify) to nudge/curve parts so that they fit better.
I found myself frequently sharpening things as I copied them in and scaled them. Especially, the seams.
Like I said earlier, we are not aiming for any cloth wrinkles here - Not yet! So try not to get too many of those. We just want a simple base to work from.
Remember to check your progress in 3dsmax to make sure you're lining things up correctly. Make sure you're getting the seams in locations that you actually like and make sure things aren't being stretched / squished.
Once the base is done, I start painting in the wrinkles.
I start by setting up my layers. I create (at least) two new layers above all my jeans and seam layers. I have a Highlights (HLs) layer and a Shades (Shds) layer. I set the HLs layer's Mode to Screen and the Shds Mode to Multiple.