Just a few tips on concealer application:

Do
-Be sure to conceal the inner eye corner as there is usually darkness there
-Use a lightweight concealer under the eyes or mix a heavy coverage with a bit of moisturizer as you don’t want to  
 emphasize any lines
-Set with a powder immediately after applying

Don’t
-Cake it on- This can actually make you look older than you really are
-Go too light in color- It can make the area around your eyes appear ashy/gray
-Forget to be gentle! The area around your eyes is very fragile, treat it with care and it will look good for years!