Stand up straight and you instantly loose 5kgs. I find the sexiest posture is a little left of the one suggested in finishing school. I prefer the stripper approach… bum out (therefore legs extended upwards), boobs out, lower back gently arched, shoulders dropped back and down. Your head should float up from the base of your neck.
This takes a little practise and no doubt not possible the whole day, but during important moments (portraits) try and ‘extend thy self’. Nothing worse than a slouching bride, and few things are as embarrassing as your wedding photographer telling you to tuck your tummy in.