From “Bridal Portrait Posing”

Getting Your Elbows Away from Your Body
If you stand with your elbows touching the sides of your body in a photograph, your body will look as wide as the elbow-to-elbow dimension.  On the other hand, if you move your elbows slightly away from your sides, creating a small separation between your body and your arm, people will notice the cut-in of your waistline in your photographs.  With your arms out, separate from your body, your torso appears slimmer.

This trick requires care in its use.  You want to see a slight separation between your body and elbows.  Pushing your elbows out too far will make you look like you are doing the chicken dance!  Also, in the course of moving their elbows outwards, many brides lift their shoulders, which we already know is a bad thing.  A little separation between elbow and waist is all that is required and all that you want.