The secret to properly applying concealer is to find the right shade and the blend it in so it looks flawless, not obvious. Concealer looks most natural when you work in "sheer layers and build it up gradually," according to the editors of Allure's "Confessions of a Beauty Editor."
To properly apply concealer, apply several dots of concealer under the eyes, then use the pad of your middle finger to tap it in (always tap, never rub). Apply concealer on other uneven spots on the face -- including the chin, and around the nose and mouth if need be -- and tap in.
Dust fine, loose powder over your face to set your makeup. According to makeup artist Scott Barnes in the August 2006 issue of Allure Magazine, loose powder applied with a fluffy brush is "the secret to good concealer."
Keep in mind concealer is not a beauty product to scrimp on (see my picks for the top 10 concealerson the market). Also, you'll want to find the right shade for your skin color. If your skin is darker in summer, you will need a darker shade for summer months and a lighter one for other months. To test shades, try them out at department store beauty counters or Sephora.