On most systems the ls command is aliased to include a number of default options. For example on my OpenSuse box I have:
$ type ls
ls is aliased to `ls $LS_OPTIONS'
$ alias ls
alias ls='ls $LS_OPTIONS'
$ echo $LS_OPTIONS
-N --color=tty -T 0
so that you can simply change the alias or the value of the LS_OPTIONS environment variable. Put the new definitions in your $HOME/.bashrc and the trick is done.