Start with Citadel Bronzed Flesh for the basecoat on all the exposed flesh (of course this assumes a human or demihuman figure). Once the basecoat has dried, wash it with Citadel Chestnut ink. You will have to experiment a little with the dilution of the ink. I've found that sometimes I have problems with the wash drying in a reverse effect (i.e., the dark showing up on the raised parts and the light showing up on the low parts). Put a drop of ink into a mixing bowl then add a drop or two of water. I find a regular drinking straw is good for this. Stir it up a bit then test it out on a patch of Bronzed Flesh that is on the cardboard. It should look pinkish.
After you've washed all the exposed flesh and the wash is dry, use Bronzed Flesh to highlight the raised parts of the face. Generally, I give the hands a quick drybrush to bring out the fingers but the face is done more selectively. I paint a bar of Bronzed Flesh down the nose and across the forehead, a spot on the cheekbones, and another line along the jawline. These are all the high areas of a face. A little bit of white can be added to further highlight the tip of the nose, the cheekbones, and the tip of the chin. Arms and legs can be drybrushed or more selectively painted. For lighter skin tones, dilute your Chestnut ink more and for darker skin tones, wash with a dark brown or Raw Sienna.
You can also play around with different basecoats. My Dark Elf army for example, is painted as Drow, with light hair and dark brown skin. In their case, I use a very dark brown as a basecoat highlighted with a lighter brown. It's up to you.
Eyes are easy but require practice. If I have a light skin tone, the first thing I do is to paint the entire eye brown. One color I've seen work very well for this is Citadel Terra Cotta. Paint the entire eye with this color using a 000 brush. What you are doing is laying down a foundation on which the eyes will be painted. Once the brown is painted in, take a 000 or smaller brush and paint a bar of white on top of the brown. You should end up with a white bar which is surrounded by the brown which will set the eye off from the rest of the face.
Once both irises are done, you are ready to paint the pupil. Start with whichever eye you are more comfortable with and paint a bar of a darker color (I prefer brown or blue) down the middle of the iris. Now go to the other eye and match pupil locations. Don't worry too much about making the pupils line up in the middle of the iris. You can get some really nice effects by varying where the pupils are and that gives your figures some character. If you want to show surprise, leave some white all around the pupil. Look at a face book or look at people and notice what their eyes are doing when they're mad, staring, looking to the sides, or trying to look innocent. These features can be repeated on your miniatures.
If a face has details like teeth etc., you can take a small brush and pick out the teeth in white. That's about as far as I go on human or demihuman faces. If I'm painting Skaven or other evil races, I like to paint their eyes red with and orange highlight for a pupil.
Monsters that have fangs or exposed teeth have them painted in a beige color first then highlighted with white. To me it just seems like it sets them out more and makes them look more natural.