If you're running drivers above version 185, then this guide is likely obsolete, as Powermizer has had a preferences option in nvidia-settings since then.
This is a nice quick one. To force Powermizer to stay at the default (and highest) clock of your graphics card always upon boot:
sudo gedit /etc/modprobe.d/optionsAdd the following line to the file:
options nvidia NVreg_RegistryDwords="PerfLevelSrc=0x2222"
Save and exit, then reboot.
Other performance levels may be specified, replace 0x2222 with 0x2233 for adaptive clocking.
UPDATE: This method does not appear to work for me in the latest versions of Ubuntu, please try the method in this post.