So, to sum it up: To make full brightness work after every boot, open the file /etc/rc.local with root rights:
ABOVE the line that reads exit 0, add the following line:
gksudo gedit /etc/rc.local
Now save the file.
setpci -s 00:02.0 f4.b=ff
This way, when you boot up, the screen will start with full brightness and you can control it normally using Fn+F5/F6 on your keyboard.
Before you do this, please make sure that for your model 00:02.0 is the correct address by typing in a terminal:
The result should be a line that reads:
lspci | grep "VGA compatible"
00:02.0 VGA compatible controller: Intel Corporation N10 Family Integrated Graphics Controller