Follow this link to see three videos of Matt Rhodes, Associate Art Director for the Mass Effect games, as he discusses the creation of the Asari, Krogan, and Salarian races respectively. I find his discussion of how he and his artists used model and animal references as launching points for their designs particularly interesting.
Designing Asari, Krogans, Salarians, and Batarians: Mass Effect: The Origin of Species – Features – www.GameInformer.com
Designing Turians: Mass Effect 3: Creating Garrus
What makes an alien race sexy and approachable to humans? What animal features make a race look predatory? Scientific and enlightened? How can designers differentiate members of these species? How would they look in different points across their lifespan? All of this must be considered when designing an alien character in addition to the practical elements of movement, weight, balance, and constructing armor and clothing.