Step 1 :
sudo apt-get install linux-tools-common linux-tools-generic cpufrequtils thermald
Step 2 :
Insert the following line to "/etc/rc.local" and before "exit 0" :
sudo cpupower frequency-set -g powersave
Step 3 :
sudo sed -i 's/^GOVERNOR=.*/GOVERNOR="powersave"/' /etc/init.d/cpufrequtils
Step 4 :
Reboot your box
Remark :
To rollback, just replace "powersave" to "ondemand" or "performance".
That's all! See you.