The biggest rule for mantle decorating is: 1 BIG ARTWORK directly in the middle. In my rental home, I broke this rule and had my SLC Temple picture about 2/3 over instead of the direct middle, with big crystal candlesticks on the other side as well as couple of smaller pictures to balance it out.
The other biggest rule for decorating a mantle that I actually do try to always follow is BALANCE. You can see I've tried to draw two triangles on either side of my mirror to show that there is balance and the eye isn't really drawn more to one side than the other and there are also things of similar HEIGHT on either side. If you can get a set of two matchy matchy things to put on either side of your mantle to have exact balance and height, then that is awesome, but it just doesn't usually work for me since I buy most of my decor second-hand/don't have that kind of money, the latter being more often the case.
For holidays and parties, another fun thing to add to a mantle is a BANNER, I've heard it called bunting, swag, garland, whatever. For Amelia's first birthday party, I made a tissue paper tassel garland that was gold pink and white and that was the only decoration in my living room for her party, and it just made it feel like a party in there because it was decorated yet it was simple - so less work for me.
Within my mantle, I always like to have little VIGNETTES to add interest.
Another great thing to have that is seriously a no-brainer is a FAMILY PICTURE. Sometimes I get so caught up in my little knick-knacks that I forget this one. But it's where people look when they enter your room because it is the focal point and you want everyone (most importantly your family duh-doy) to know that your family is important.
If you can add FRESH FLORALS to your mantle, more power to you - you can now call yourself Martha Stewart for the day.
Now, if we ever decide to mount the TV to our fireplace to be able to rearrange the furniture in my family room how I want it, I plan on finding a thrifted mantle for our bedroom to be able to decorate to my soul's content. Can you plan on finding things thrifting? Probably not, oh well. My mind is always churning out new decorating ideas to fill my house.
Hope you enjoyed, now get decorating!