Eyeliner. Notice how there's a hint of blue on her lower lids...and she has blue eyes. Traditionally you go opposite in eye makeup colors (blue eyes get neutral colors, brown gets jewel tones), but I have blue eyes and do this on occasion. It really makes them pop!
Whether you're just starting to figure out the wonderful world of makeup, or have been coating your face with it for years, these tutorials are filled with fantastic tips and tricks to teach you how to apply eyeshadow PROPERLY.