/etc/sudoers
Because the syntax of the /etc/sudoers file is obscure, I'm noting down here exactly what I've done to mine:
root ALL=(ALL) ALL
%admin ALL=(ALL) ALL
%admin ALL=NOPASSWD: /usr/bin/wodim
%admin ALL=NOPASSWD: /usr/bin/apt-get
%admin ALL=NOPASSWD: /sbin/reboot
%admin ALL=NOPASSWD: /sbin/poweroff
This allows a few commonly used commands to be executed without a password.