Ever heard that observation that you need to choose between your face and your backside, at some point in life? (Thin = skinny backside, but also hollower face - where a little extra weight can be flattering.) According to Dr. David Orentreich, Clinique's guiding dermatologist, it's particularly important not to yo-yo diet in your 30s and beyond, because you'll never gain the weight back in your cheeks.