Automatically add current user to the sudoers file

if sudo grep -q "$USER ALL=NOPASSWD: ALL" /etc/sudoers; then
   echo "passwordless sudo already active"
else
   echo "setting sudo without password for $USER";
   sudo sh -c 'echo "'$USER' ALL=NOPASSWD: ALL" >> /etc/sudoers'
fi

Logout or Reboot

sudo logout

sudo reboot

Doing the above Steps Manually

sudo visudo
[sudo] password for a:

#sudo vi /etc/sudoers
# file above is read-only, only do below

sudo visudo

# add the follwing command
a ALL=NOPASSWD: ALL

# https://help.ubuntu.com/community/RootSudo
# Type in ^x to exit. This should prompt for an option to save the file, type in Y to save.

Logout or Restart Server

References

1. http://askubuntu.com/questions/147241/execute-sudo-without-password