I have a somewhat-quick but effective way I like to use:
Paint the face the same colour as the rest of the skin (in my case, Tallarn Flesh for a basecoat), then wash with Ogryn Flesh. Layer with Dwarf Flesh, leaving the darker shade in the recesses.
Then, focussing on the T-Zone:
http://2.bp.blogspot.com/-SudMXducyFo/T_7s6E92nZI/AAAAAAAAC7Q/5KlboFIDT0M/s1600/tzone.jpg (yes, we're getting into makeup terminology... but then, what else do you call painting faces?

), highlight with elf flesh.
For the eyes, I just paint a base of Skull White, then add the pupils in Chaos Black.
Note: One side of the pupils should touch the edge of the eye... top sides, whatever, it doesn't matter as long as you don't leave a dot isolated in the middle of the eye, otherwise the mini will look startled, as though they've just been goosed.
For my guard, I also give a wash of red on their noses- peoples nose tends to be a different colour to the rest of their face, but it's very much a matter of taste. ;p
A (slightly) blurry example: