Depending on how AU it is, you can use the military history of WWI for a base. Also, the Spanish-American War, although it was right at the end of the 19th century, should hold some good resources. The Canadians have an awesome WWI repository, with a great oral history section -- you can listen to these old men describe their daily life in the ranks and the battles they were in.

First Readers are an invaluable ally for me in keeping me on track and for bouncing storylines and ideas around. They're not for every writer, but it works for me. :-)