Decorating a Fireplace Mantel takes practice or at least I think it does. It has to feel just right when putting your objects up there. It doesn't always have to be symmetrical-meaning the same on all sides, it can be asymetrical or different on the entire surface, but the heights of objects need to vary and the size of objects has alot to do with it.
I first take into consideration what I am feeling at that moment: color, texture, lighting. I enjoy having the little white lights up there so that in the early evening, rather than having an overhead light on, I can have a little twinkle in the kitchen along with a candle lit on the counter and it is super cozy.
My mantels in my home are on the smaller size because they are all faux or fake mantels and not real built in fireplaces. I gather my objects and always try to have a back drop behind the objects on the blank wall. If you walked into my home, you would notice I have trouble keeping a wall blank. I have one right now in my living room that is blank, ready to be primed and painted, and I just can't determine if I like it plain or not. So anyway, get a painting, a mirror, a plaque, a series of dishes or photos and hang them up first so that you are not trying to adjust your wall and break any of your precious finds. After you have your wall or mural done, then start placing your pieces on the mantel. Use books, perhaps cookbooks if it is in your kitchen, use glass, wood, pottery, perhaps some flowers in a vase, a candle placed (always be careful of those candles:) perhaps an odd vintage looking gadget, tuck a little picture frame of one of your children, yourself or a relative in between the pieces.