When I first moved back to Florida some years ago now, I went to an early morning photo review with a local photography Meetup group. I brought my print portfolio with me and the piece of feedback I received that stuck most in my mind was that I had no shots in portrait orientation. My print portfolio was of course a very small sample of my work, but from then on whenver I went out to shoot I had in my mind think about shooting in portrait orientation. This means holding the camera vertically, rather than the more natural horizontal (landscape orientation) position.
Since that time my photography experience has greatly increased and I have been teaching photography lessons for years. In those lessons I see the same thing with many students, almost never shooting in portrait orientation. One way to start to train yourself to shoot in this orientation more is to first try and photography vertical subjects in it.
In my above photo of a flag pole, the tall nature of the subject naturally fills the frame from top to bottom when shooting in portait orientation. I chose to make the subject off center as well with the flags blowing into the empty (negative) space. Had I made this shot in landscape orientation, there likely would have been too much negative space resulting in the subject getting lost in the frame.