One of the easiest ways to decorate your mantel is to start with a big piece and center it above the fireplace. Pick something you love — whether it's a mirror or a piece of art — and then base the rest of your design around that.
Play with movement by adding in light and height — candles and items of differing heights allow the eye to move up and down along the display. Add visual weight to the display with a secondary focal point, like a vase or sculpture. This will relieve the weight of the other pieces.
Starting with a single large object achieves a look similar to the three-plus-one rule — a visual triangle that allows the eyes to travel up and down, creating a display that you'll never tire of.