Choose a focal point with a curved top to mimic the arch of the mantle. A decorative half-moon stained glass window or arch-top mirror might fit the bill. A half-moon window or piece of arched vintage architectural salvage mounted above a rectangular piece of art plays up the curve of your firebox or mantle trim as well. Warm colors evoke the shades of the fire below and cool colors contrast the shades of the fire.
Position two decorative accessories such as vases, candlesticks or framed pictures to the far right end of the mantle. Whatever items you choose for this area -- mix and match for a more casual look, or choose symmetry for a formal style -- they should have straight up and down lines. The straight lines contrast the curve of the stained glass.
Position two different and petite sized decorative accessories such as statues, jars, pitchers, clocks, and other items on the far left side of the mantle shelf. These items should have straight up and down lines and be the same color like the items in step two, but should be shorter than those items. This way you're evoking the straight sides and arched top of the firebox below with this display of slightly asymmetrical display configurations.