there is some serious Victoria Beckham type discipline going on there
Or smoking 
TBF, I've always agreed with Elizabeth Hurley who said, in response to criticism about how quickly she got back into shape after giving birth, that it is her job to wear clothes and look good in them. Kate's "job" is clearly a bit more than that, but how she looks is a major part of it (as these thread attest!)
The lilac dress was beautiful in its simplicity and perfect fit. I used to have a vintage Jaegar dress that fitted me like that - I'm too, ahem, broad for it now unfortunately