Once upon a time, I didn't even think about wearing concealer.  I felt I didn't need it and had no idea how to apply it properly.  Now it's one of my most used products, but it's definitely a double-edged sword.  When applied properly, you look more youthful, awake and refreshed, when applied improperly, however, you can look totally crazy.  Here are a few tips I've picked up along the way that have helped me to avoid looking like this. 1. Moisturize appropriately.The skin underneath the eyes is delicate and often dry.  To make sure that my concealer spreads smoothly, I make sure to apply an eye cream and then wait for it to absorb before applying concealer.  I'm currently using Clinique Superdefense Age Defense Eye Cream, but have heard great things about Bobbi Brown Hydrating Eye Cream.  If the skin is properly moisturized, the concealer will apply more smoothly and you can avoid dry patches and flakes.2. Match your skin tone (usually).If I'm not wearing foundation or tinted moisturizer, then I apply a concealer that matches my skin exactly, and then I apply a very light dusting of loose powder to set.  Our faces naturally have variations in tone, so if I'm not applying a foundation or tinted moisturized to even everything out, then I will use a concealer to accomplish that.For this, I use YSL Touche Eclat Radiant Touch in No. 4 (very sheer and easy to blend), NARS Radiant Creamy Concealer in Amande (creamy and quite opaque), Laura Mercier Secret Concealer in No. 5 (salmon undertone that really brightens and very moisturizing) or Chanel Correcteur Perfection Long-Lasting Concealer in No. 42 (very easy to blend, opaque and dries into a nice matte finish).3. Or, go a shade lighter.If I'm wearing foundation or tinted moisturizer, then I will apply a concealer one shade lighter than my skin.  I do this because once foundation is applied, my face is one shade all over, but this can actually look less natural because that is not how our faces look.  By adding a concealer that is lighter, you can restore some depth to your face by doing some highlighting with the concealer.For this, I use YSL Touche Eclat Radiant Touch in No. 5.5, MAC Mineralize Concealer in NW 35 (great highlight and long-lasting), or MAC Select Moisturecover in NW 30 (just slightly lighter than the other MAC concealer and more moisturizing).4. Apply lightly.This is the biggest lesson that I learned.  Apply the smallest amount possible (either using your ring finger or a damp beauty sponge) and let it dry.  If you need more coverage, apply a second light layer.  I've learned that using less decreases the likelihood of the dreaded creases.5. The triangle.I apply concealer in the shape of a triangle.  It looks more natural and highlights the whole under-eye area.Here are some swatches of my favorite concealers.  Note that there's quite a range of shades depending on what other makeup I'm wearing.What's your favorite concealer?