1. GENES. Mom test does not always hold up, but it is genes, so that is a good start: good skin, hair, teeth; narrower hips, slender legs, moderate or even smaller breasts (if you are questioning this think about how many women with huge breasts or augmentations like to run or excercise...), normal or desirable proportions and facial features (nose, lips, distance between eyes). Check out the dad too, it's harder to picture, but that is an enormous factor.
2. CHILDHOOD. Do a good study of her upbringing, this is not only a mental health/education factor which will help you do the distance, but attitudes about sex, diet, skin care, fashion, excercise, etc., it all happens here. Too late really to modify most of these.
3. LIFESTYLE. Lifestyle eventually plays a major factor but if you have observed French women much, you know that excercise does not count for much in terms of size and shape (go back to #1 & 2), only muscle tone. Diet and excercise are of course going to help but that is more importantly a reflection of intelligence and early family life, attitude, and psychological well-being. Beware, there are tons of 40-something psychos starving themselves and doing 2hr workouts...you don't want that either. Lifestyle is the most controllable post child-rearing, so this can evolve if #1 & #2 are solid, even if things change a little during the wonder years.