You can create your own "definitions.ly" file as described in the
Learning Manual here:
http://lilypond.org/doc/v2.11/Documentation/user/lilypond-learning/Style-sheets#Style-sheets
Then you just have to put \include "definitions.ly" at the top of your
documents.
I like to put the definitions file in my $HOME directory because I
found that if I put it in the directory with the other lilypond
program files it kept getting deleted when I upgraded lilypond. If
you put it in the home directory, be sure to include the home
directory in your lilypond command:
lilypond --include=$HOME filename.ly
This is the command on Linux anyway. I don't recall which OS you're
using. You'll have to adjust if you're on Mac or Windows (actually I
have no idea how to do this on Windows...)
Jon