I did the art work for Bungie's first biggish hit Pathways before deciding to go off and get a Ph.D. My artwork at the onset wasn't great but got progressively better with each successive series of animation frames I drew (and redrew). I spent a lot of my time getting people to pose for me, looking at how animals moved (being a biologist with access to numerous species helped), and trying lots of different combinations of features before deciding on a finished product. Be prepared to go through a lot of paper, and try to think like an artist all the time, studying the world around you for ideas. Also, steal liberally from other artists. They have all done the same.
By the way, for anyone interested and who read the Bungie history recently posted, here's a slight addendum. I did the original artwork for the aliens that appeared in the beta version of Marathon (and mine were MUCH more alien looking- actually criticized by Alex and Jason as being too alien). But because it had a certain sameness about it (similar artistic style), everyone just thought of it as a sequel to Pathways. So a new artist was brought in to give it a new flavor. They also scrapped my idea of having the whole thing occur in a spherical ship with interlocking layers, thinking that might be to challenging for the more linear minded players.
Cheers.
p.s. By the way, I am quite amazed that anyone is actually interested in this stuff still, but more power to you.
