Firstly, it must be noted, that to put a correct posture can hide the defects. Basically, it is not recommended to let the arms and legs spread out straight, but to put the arms and legs in overlapping position.
If specified, the model has to put the arms behind the body or cross arms, while the legs are moving cross standing on it. Because the crossing / overlapping legs posture, often appears in fashion magazines. Thus, the model would not be surprised and very easy to put a good-looking stance. This posture also has the benefit of people with O-leg stand. It can help to hidden the O-leg shape instead of a beautiful effect of overlapping / crossing posture.
Sometimes using Tilt-shift lens and other auxiliary equipment, managed to look more uniform body proportion. If you adjust the lens optical asix movement, the legs will look more slender.
Or you can use wide-angle lens when shooting from the bottom up, head looks small, and the model's legs look more slender.