Big and tall, or tall and lean... These are the rules of accessorizing a mantle.
Too many little things will get knick knacky. To avoid a cluttered mantel, you're going to want to make a few bold gestures. Large vases allow you to dress the mantle seasonally with live branches, holiday boughs, pussy willows, etc.
Large art or a large framed mirror is most commonly centered above a mantle. In Feng Shui it is believed that the mirror will bring the energy back to the room, since the fireplace is a hole in the house where energy can escape. It is true that mirrors bring energy and light to a room. One of the great designers, I believe Suzanne Kasler says, "A mirror is like adding water in any room."