I found this article extremely helpful as I used to frequently wear my shirt untucked and as mentioned it just felt plain and frumpy.
Tucking in my shirt and adding a belt does break up the midsection and add positive focus to my body and outfit, especially with the high waisted jeans that cover our belly area. Empty belt loops bother me so this was an easy and fab solution, but for untucked shirts wearing a belt over the shirt also works.

My only question is what determines the “third” line if we all have different torso and leg lengths, and how to figure it out whether we have high waisted, mid rise, or low rise jeans on as this changes how it splits our body. (See photo below)

Be sure to watch the video at the end of the article!
http://blog.adacollection.com/.....nd-a-belt/