Watch the amount of "top space" on both horizontal and vertical compositions. Deliberately scan the entire frame, especially the areas that are "around the outside of" your ostensible subjects. When the background is simple, and clear, and free of Sasquatch-walking dudes, then shoot! But when the photo-bombers are in the background area, DON'T shoot. Keep at it. Keep shooting.
The tendency is to focus in on "just" the person you're photographing, and to allow unwanted stuff to creep into the frame area. Making a very deliberate, focused effort on scanning the entire area AROUND the main subject will cure this issue fairly quickly, like maybe six months.