Frame the face. So choose clothes that so just that. A v-neck, rounded, or collars work well to help to do that turtle necks do not. Like my portrait tips on what to wear passport photo tips are no different so stick to no strips or patterns. Dark or neutral colours work well but you can add a scarf to help add a little colour to or your passport photo.
As for the woman out there, what to do with your hair? Up or down? Down. Bringing your hair down help with the basic rule of framing your face. But if you are a man with long hair please tie it back.
As for jewelry nothing too overly big but nice small simple pieces can add a nice touch.
Make up should be a slight and I mean slight heaver then daytime makeup but not as heavy as nighttime makeup. Colour on your lips can really make a difference in pictures so at the very least try some colour there.
For the men out there please the lip colour is not necessary