Ensure that the exposure is done in deep suspended inspiration. There should be 6 1/2 anterior ribs seen or 10 posterior ribs seen.
Not standing straight:
If the patient doesn't stand straight i.e the coronal plane is not parallel to the film, either
a) the patient leans back or
b) leans forward,
the image will not be satisfactory as above.
The scapula will be in the lung field if you fail to rotate the shoulders forward and down.
If the patient is rotated, the median saggital plane will not be at 90 degrees to the film. This will be shown as asymmetry in the distance of the medial ends of clavicles from the spinous process.