I have found rpgmaker's solution to keeping laptop brightness in Linux Mint. http://askubuntu.com/a/227553

Just to keep it safe, following is a backup of the original article

Original url: http://askubuntu.com/users/116453/rpgmaker

A small script saves last used value of brightness (stored in file BRIGHTNESS_CONTROL) to file (SAVEDFILE) on shutdown and restore it from file on next startup. If there is no such file - script set it do default value (DEFAULT_LEVEL). I placed this file in /etc/init.d/brightness:

#! /bin/sh
### BEGIN INIT INFO
# Provides:          brightness
# Required-Start:    $local_fs
# Required-Stop:     $local_fs
# Default-Start:     S
# Default-Stop:      0 6
# Short-Description: Save and restore brightness level between restarts.
# Description:       This script saves the brightness level between restarts.
#                    It is called from the boot, halt and reboot scripts.
### END INIT INFO

PATH=/sbin:/bin
SAVEDFILE=/var/lib/brightness-level
DEFAULT_LEVEL=4
BRIGHTNESS_CONTROL=/sys/class/backlight/acpi_video0/brightness

. /lib/init/vars.sh
. /lib/lsb/init-functions

do_status () {
    echo -n "Current brightness level is `cat $BRIGHTNESS_CONTROL`"

    if [ -f $SAVEDFILE ] ; then
        echo ", saved value is `cat $SAVEDFILE`."
        return 0
    else
        echo ", there is no saved value."
        return 4
    fi
}

case "$1" in
  start|"")
    [ "$VERBOSE" = no ] || log_action_begin_msg "Initializing brightness level"
    # Restore brightness level
    if [ ! -f "$SAVEDFILE" ]
    then
        echo "$DEFAULT_LEVEL" > "$SAVEDFILE"
    fi
    cat "$SAVEDFILE" > "$BRIGHTNESS_CONTROL"
    ES=$?

    [ "$VERBOSE" = no ] || log_action_end_msg $ES
    ;;
  stop)
    # Save brightness level
    [ "$VERBOSE" = no ] || log_action_begin_msg "Saving brightness level"
    cat "$BRIGHTNESS_CONTROL" > "$SAVEDFILE"
    ES=$?
    [ "$VERBOSE" = no ] || log_action_end_msg $ES
    ;;
  status)
    do_status
    exit 0
    ;;
  restart|reload|force-reload)
    echo "Error: argument '$1' not supported" >&2
    exit 3
    ;;
  *)
    echo "Usage: brightness start|stop" >&2
    exit 3
    ;;
esac

:

This file is based on /etc/init.d/urandom. You can customize the BRIGHTNESS_CONTROL and DEFAULT_LEVEL for your needs. You can test it from console after running sudo chmod +x /etc/init.d/brightness

• /etc/init.d/brightness status (show current brightness level and saved in file value)
• /etc/init.d/brightness start (set saved level from file)
• /etc/init.d/brightness stop (save current level to file)

For normal operation of the script you need to create symlinks for 0 (shutdown), 6 and S (startup) runlevels.

ln -s /etc/init.d/brightness /etc/rc0.d/S25backlight
ln -s /etc/init.d/brightness /etc/rcS.d/S25backlight
ln -s /etc/init.d/brightness /etc/rc6.d/S25backlight