Add this to /etc/rc.local above the 'exit 0' line (e.g. sudo nano /etc/rc.local from the terminal):
echo OFF > /sys/kernel/debug/vgaswitcheroo/switch
And then add this to /etc/modprobe.d/blacklist-local.conf, also as sudo:
Reboot. Verify by doing the following:
sudo cat /sys/kernel/debug/vgaswitcheroo/switch
You should see something like this:
IGD is the Intel chip, the + means it's the one in use, and Pwr means it's on.
DIS is the ATI card and if it says Off, you're saving a ton of battery.