Undervolting is able to give the most drastic across the board reductions in power. Do you have a power monitor? They're not very expensive and are very handy for this work and also monitoring household appliances too:http://www.maplin.co.uk/plug-in-mains-p ... itor-38343
My order of doing things:
1. Remove any hardware that is not required. For example do you need a power LED?
2. Set things at the jumper level on the motherboard if required, some boards have a jumper to enable/disable undervolting and clocking in the BIOS.
3. Disable everything in the BIOS that is not required. Extra controllers may use a tiny amount of power.
4. Reduce voltages in the BIOS to reduce power consumption, stress test quickly to make sure system is still stable. If it fails to boot, most BIOSs will revert changes when you restart.
5. Then look at what may be possible in software, this is very hardware dependant.