I think the key to faces is patience. Eyes, especially.

Start with a base coat of your flesh color and make sure it is even and consistent. Then pick another color (dark purples or reds work well), thin it down a lot, put your brush in, remove the excess on a paper towel and then pull the paint from the high spots to the recesses and sides. Here's an example from a WIP of my orange birdmen.



Here's a video of a good way to do faces by layering the colors on top of each other: http://www.youtube.com/watch?v=-ac1oLYiyi8