Available light photography is a perfect way to capture many portraits, but sunset is not a time to utilize this option. Without flash you either have a silhouette or muddy skin tones along with a blown out sky. Solution?  Add a flash to bring out the details. On this portrait I used a Canon speed light 580ex2 flash off camera, a cybersyc  flash trigger along with a Canon 580 ex2 with a Gary Fong diffuser on camera for a little fill.

ISO 100/ F8 aperture at 1/250th shutter to maintain detail and color in the sky.