The trick is to find a concealer that matches your skin -- it probably won't be as close as your foundation because concealers simply don't come in as many shades. It works best to apply concealer on top of foundation, because you'll see exactly where you need a little extra coverage and you won't smear it off as you pat on the foundation.

A flat concealer brush is a handy tool for applying it, but you can use your finger, too. Remember, a little goes a long way, so gently tap a bit where you need it (this means underneath the eyes, a touch on the nose to take out redness, and the odd spot to cover any blemishes.) For extra staying power, dust on a little bit of translucent powder to set the concealer and you're good to go.