When taking full length portraits, do not use a straight on flash. This dulls the portrait. Try to bounce the flash off of a surface. This produces a calmer portrait with pleasant lighting. Natural light is best to use; however, it may not be suitable in a studio setting.
If you take the portrait outside, avoid the afternoon light. It is best to take outdoor portraits in the early morning of during the evening hours. If you are taking an outdoor portrait, avoid a strong back lighting behind the subject. This will cause their features to become shadows. This will also cause their face to become distorted and their eyes to be hidden.