Step one: be born with the genes to have a frame like hers.
Step two: get willpower of steel.
I'll tell you a little story. I was working at Jil Sander S/S 2011. We were all backstage for hair and makeup. It was Daria Strokous' birthday and they surprise her with flowers. How sweet, I thought. But then they came back with a cake!
A few of us took small pieces but (using makeup as an excuse) had one/two bite to be polite.. Vlada not only didn't have any, but she
held Daria's plate for her on her own lap (Daria had no place to put the plate, and was holding her flowers) while Daria had some (not much, but some). And Vlada just stared the cake down... like she was a big magic genie and it was some evil little dwarf. When Daria was done, Vlada calmly walked over and put the plate with the rest of the cake on the tray.
I've seen her do stuff like this, it's like she enjoys testing/tempting herself...
BTW, I've seen pictures of the "cake incident" online.. I'll see if I can find them.