How to undervolt

First a warning: don't select Set on boot until you are completely sure your processor is stable with the voltages you selected. You can find the voltages settings under System settings > Performance > Processor. The extended settings automatically reset Set on boot to prevent accidents. My advice is to undervolt the lowest frequencies only. This will save a little battery without too much risk for instability. Both my devices (Xperia Pro and Xperia Mini Pro) are 100% stable with the values below, but this can differ for each device type and even for each individual device.

• 24 Mhz: 750 mV
• 61 Mhz: 775 mV
• 122 MHz: 775 mV
• 184 MHz: 800 mV
• 192 MHz: 800 mV
• 245 MHz: 800 mV
• Other frequencies: default

For some background information, see here.