1. In a group shot, the one standing closer to the camera will look the biggest (read fattest). So rather take a step back to look the slimmest in the group.
2. Hold your breath and pull your stomach in. This will make your posture more upright and you will definitely look slimmer.
3. Wear your best possible heels. This will add height and you will look slim. Stilettos to be preferred. Wedges and platforms will make you look bulky.
4. If you are too broad, give a side profile.
5. If you have double chin, make sure that you do not drop your head down. It will highlight the double chin. Look upward. Also make sure that the camera angle is not down below you.
6. If you have flabby arms, make sure, they are not pressed against your body or wall. Keep them away from you. Hands on your waist/hips can be a good option.
7. How you stand also makes a difference. Do not stand with wide legs apart, with weight on both feet. This will definitely make you look broader. Either cross your one leg over the other or shift your entire weight on one leg and extend the other out. The previous pose makes you look slimmer and the later gives and extension to the leg and hence there is more definition in your profile.
9. Try not to blink or stare to hard into the camera. Where you look, is really important. Is it right into the camera or looking away a little. Is it looking down or up. Different angles demand different looks.
10. See whether it is close/tight shot or a long shot. Accordingly take give an angle.
11. Do not stand against the light. The light should be on your face.
12. Last but not the least- remember- stomach in, chest up, arms away, chin up, smile, turn a little side ways and go for it. Do not be too self conscious. Play around and pose like a professional model.