After studying my own skin for some time (I am the stereotypical pale-skinned, freckled, green eyed redhead), I tried painting layers over a skin texture. One layer, several freckles of a certain size/color; second layer, a slightly different size/color... several layers and then one for the odd parts like moles and skin blemishes. Blur each layer a bit, add some noise and voila! fairly servicable freckling.