Most people who have fireplace mantles, for the most part a majority, put the TV above the mantle which is fantastic. It saves for you having to move the furniture to the TV,then focus the fireplace. Which one do you do? So it's two birds with one stone, right? The problem is then you have this really boring mantle. You have nothing above it, because you want to see the TV.
So how do you decorate it? Well, one way you can do is objects on either side of the TV. So you can have candles go on either side or vases go on either side. Another thing that you can do is to do something really low. I love doing smaller candles on the sill right below the TV. You can also do a wreath if it's the holidays, and you can do something that has really beautiful, floral and lush.
Remember if you're doing groupings, like even a small grouping, the taller one of the three can go just a little higher than the base of the TV, because you have a lip around the TV that covers about two inches. So it doesn't just have to stop just before you hit the TV. Remember that you can fudge it a little bit.
Fresh flowers are always easy for me, because you can put those in small vases, tiny ones. You can do them in big ones. You know, family photos, just as long as it's a little away from the center of the TV. You don't want people concentrating on what's on the mantle more than what you are watching on TV, because it still has to be all about function.