Set both ambience and diffusion to the texture. Diffuse should be 100 (unless you want darker colouring) and ambience 12. Specularity is only needed if you want the skin to look wet for some reason; settings of 5 to 10 are appropriate. A bump map with a strength of 2 or 3 can be used to give more detail. It's better to use a proper bump map than a sandy texture in the bump channel.