The most obvious is to decorate with items that start conversations! Use items that mean something to you so you cam talk about them. Also, use an odd number of items in groupings since our subconscious tends to pair things up, which can be very boring.
Also, use color to attract attention to your mantle - something that pops out at you, but still works with your color scheme.
To build or decorate? Here are ideas on building one: https://www.hometalk.com/search/posts?filter=how%20to%20build%20mantel