Ensure the skin looks real
A common mistakes I see in student work is flat, lifeless skintones. Skin has a lot of subtle shifts in colour, especially where capillary vessels are near the surface. Using an Overlay layer and a desaturated red-purple, I brush in colour around the ears, nose, lips, cheeks, hands, feet, elbows and knees. Be careful not to overdo it though, or your character will look like Rudolph.