For sitting down in posses during your session it is normal to want to sit comfortable, I can't tell you how many times a friend will come with a friend for support during the shoot, and that is great but the friend will yell out "you don't look natural sit comfy and sit how it feels natural to you".
Let me tell you nothing about modeling is comfortable it should be work you pull the natural look off with your expressions and the way you present yourself. If your sitting doesn't mean it time for you to slack and recline back... try and put your weight on the back of one thigh, rather than distributing your weight equally on both. This will help with the slimming effect.