Understanding what your facial expression looks like in everyday settings is really crucial to looking warmer and becoming more likable. I consult with a beautiful female physician at a major Chicago hospital whose boss told her she needed more charisma, and that her patient satisfaction scores needed to go up. Upon getting to know her, she was lovely, but when you first met her, her piercing blue eyes and serious countenance made her appear downright icy.
During our first meeting together, I asked her to describe her job while I was videotaping her. Upon viewing the footage, she had tears in her eyes; she was really shocked at how cold and aloof her facial expressions were.
Next, I had her do a second video, describing her job, but smiling the entire time (even while she was talking which feels super awkward). The simple change of smiling changed everything. She couldn't believe how much lovelier she looked while smiling - and how easy it was to do. The work results were staggering. Her patient satisfaction scores climbed rapidly above 90 percent, which she was thrilled about.
To do this at home, you can have someone videotape you, and practice until you find an expression/style that you like, or even just practice alone in the mirror, which is also very effective. When you find an expression you like, your face will develop muscle memory for it, and you'll be able to put it on your face pretty easily and know how it looks.