I don't make skins (yet), but I teach drawing and painting IRL, so I may be of some help.
The skin tone looks too beige and too smooth, it looks like a woman with too much foundation

. I don't know HOW to do this, but the best skins I own have imperfections, slightly irregular skin tones, like in real life.
It looks like your shadows and highlights were done with black and white airbrush strokes. This always looks unnatural, real life shadows and lights are never pure black or pure white. In a "real" painting, adding white or black to light or darken a paint makes the color muddy or washed out, we use complimentary colors for shadows, and brighter paints for lights. In photoshop, the easiest and best way to light/shade a painting is with the dodge/burn tools. If you don't want to "hurt" your base drawing, you can use a 50% gray overlay to draw your shades and highlights.
The eyeshadow is too strong and bidimensional, and it seems you haven't drawn the eyes behind it. Even if you really want dark black eyeshadow, I think you must draw the eye folds and make subtle highlights to add dimension to the eyes. Blacks are never a pure, zero light, unsaturated black. If you look close to a good painting (even a photoshop painting), you'll see they're more like dark grays with some warm ("brownish"

or cool ("blueish"

undertone.
A good point: I think the lips are very well drawn

. They still look a bit too bidimensional for me, but just a little highlighting on the right places would fix it
When you're drawing from scratch, if you plan to have realistic results, it helps a lot to have some reference. Ideally, IRL, a live model

. But it may be a picture, or even a mirror.
I'd like to see a picture took from the side of the face. And I think it would help if the skin was on an shape with smaller lips. You have lovely lips

, but I think it would be easier to judge the skin quality with a more average sized avatar.
edited because my english sucks