I was born with them, can't really hide them as they are also a crease. The skin under and around your eyes is thinner and the blood vessels closer to the surface, so swelling seems more pronounced. (for me, swelling is a godsend-the creases aren't as noticeable!)
Keep the area moisturized as well as trying to conceal it. I use eyeliner under my eyes, in addition to adding a bit of depth (I have hardly any lashes at all there), it draws some attention away from the circles-just don't make it really dark liner or you'll look like you have crop circles there. I also use a flesh-toned eyeshadow to keep the line thin and from spreading. Never use a concealer lighter than your face. I have also seen on (I think it was) Oprah, concealer should go on LAST not first, then blended into the rest of your makeup. Finish off with a good powder if that's your choice.
And if none of our suggestions work, there's always tinted glasses
