Many Bibtex styles support control over indentation using the
bibhang dimension, although I haven't found an alphanumeric style that supports it.
The code below shows how support for
\bibhang can be added to vanilla Bibtex, so that it can be used with
%% Absurd indent to show off what is happening
%%Redefine \bibitem to ensure bibhang is used
%%This could conflict with packages that want %%to change these parameters