One thing I can not do without is the collection of books I have acquired over the years to help in the research of animals. I don't care how many pictures you take or sketches you draw, when it comes down to the final clay you want to have a reference that is laying right in front of you and is anatomically correct.
My favorite two books to constantly dog ear or turn to are Animals in Motion by Eadweard Muybridge and The Atlas of Animal Anatomy by Ellenberger and Baum. Animals in Motion take you through high speed shots of 34 different animals, action pictures frozen in time that really give you the ability to see muscles in action.
The Atlas of Animal Anatomy is interesting because it breaks up the animal anatomy to where you can see the development of the animal from bone to hide. That gives you the advantage over someone that does not even look at the influence of bone and muscle under the skin of an active animal. The only way to really do an animal correct is to understand the anatomy and then you have to understand the animals personality and traits. Posture is impossible without structure.
I also haunt the library sales where often I can find old books with either information that I need or pictures that give me further incite. You not only use as much out in the field observing that you can but you also find as many other sources to aid you as possible.
Two subscriptions that I adore are Ducks Unlimited and North American Hunter, I know they are meant for hunters which I am not but the information gleaned from them is immense. Even though I learned about ducks picking up dead ones for the Division of Wildlife from a botulism die off I still caught myself referring to photos in Ducks Unlimited.
Hopefully as a patron you will appreciate the trouble most artist go to in order to develop the skill needed to capture a moment in the life of a beautiful animal. I never even knew elk could jump gracefully from a standing position until I witnessed it myself...little things bit by bit collected become a work of art.