I would recommend that you buy and read Chapter 5 in "How to Build Dioramas" by Shepherd Paine (1st version, yellow cover) or Chapter 4 in the 2nd version of the book. It has some very good techniques and pointers. There is a large color illustration/example of the step-by-step process of painting faces (page 43 in version 1 or page 59 in version 2) that I found very helpful and use as a basis for my technique. For the first few figures, I kept the book open next to me for quick and easy reference, but now that I have the technique down, I can go from memory and adjust as needed for each figure.
I use Model Master enamel paints and a fine tip brush, and a wide brush for blending (use soft, bristle, high-quality art brushes; they may cost a little more, but well worth it in the long run).
1) I start by underpainting the figure's "flesh" areas with a thorough coat of Dark Tan. I usually wait at least 24 hrs to let the paint fully cure, that way it doesn't easily wear off during the follow-on painting and blending.
2) I mix the Dark Tan with a little Flat Rubber (black is too dark) to make the shadow color and paint the shadow areas (eyes, under brow, under and around nose and mouth, under chin, behind ears, shadow areas of hands, between fingers, etc).
3) I clean the brush and then use Flat Tan to paint the highlight areas (brow, ridge of nose, cheeks, upper lip, chin, and outer area of ears, highlight areas of hands, fingers). Do a little at a time and immediately lightly blend these highlights with the wide brush--kinda like drybrushing, but the paint is already on the model.
4) I clean the brushes (make sure you get them fully dry, or you'll clean the paint you just applied right off), then use Flat Light Tan and paint the light highlights (brow just above the eyes, ridge of nose, cheeks, upper lip, tip of chin, knuckles and fingers especially tips). Once again, do a little at a time and immediately blend these with the wide brush.
5) Paint the eyes. I use Model Master Acrylics for these because they are easier to clean up if you make a mistake. I use a sharpened round toothpick to apply Flat Light Gray for the whites of the eyes. Once that is dry (usually 5-10 minutes), I use a different sharpened round toothpick to paint the dark of the eye (iris). Since eye color is pretty hard to see at 35 feet (1/35 scale), I just use Flat black. Wipe your toothpick clean after each use, otherwise the paint will build up and you'll get a huge blob of paint exactly where you don't want it--or one eye will be bigger than the other. Immediately use a sharpened toothpick to "shape" the eye and the iris as needed. If you make a mistake, or it just doesn't look right (no pun intended) a toothpick works well to wipe the slate clean so you can try it again.
7) Now paint the hair and eyebrows using whatever colors you desire. Just don't forget the highlights and shadows. This is also the step where you add beards, facial hair and 5 o'clock shadow. Once again, using Enamels, I use a Flat Tan and Rubber mixture, maybe darkened with a tiny dot of flat black as desired. Paint the 5 o'clock shadow, and don't forget to the add shadows under the chin and jawline (that's where the black addition comes in handy).
Let the flesh dry for at least 30 minutes and then I usually paint the clothing next. Just be careful not to handle the flesh paint too much or you'll wear it off with the oils in your skin. Also, care must be taken when painting around the collar and cuffs so that the clothing color does not bleed over to the flesh. I seal the figures with Testors Dullcoat spray, and haven't had a problem yet. Hope this works for you! Enjoy and keep us posted.
P.S. I'll post pictures of examples of my successful results as soon as I can borrow my buddy's digital camera or buy one of my own.