Step 1: Windows Energy Saving Features
Windows has built in features to save energy
Go to the control panel, and click "Power Options." Select a power saving power plan. Then go to "Choose When to turn off the display." My personal preference is to turn off the display after five minutes idle, and sleep after ten minutes.
Step 2: Physically Reducing Power Consumption
Downlclocking your components reduces the speed at which they operate, but will also reduce power consumption and head production. You can downclock your video card with software like ATItool and EVGA precision tool. Contrary to it's name, ATItool also works on nVidia cards.
EVGA precision (for evga cards only)
Step 3: Other
Lower the brightness on your monitor when it is dark out.
Buy an 80+ certified power supply. These guarantee 80 percent efficiency at many load levels.
Here's a simple one: Don't leave your computer on when you're not using it!
There are other green components you can buy such as hard drives.
The new HD 4890 video cards automatically downclock themselves when they idle, and use significantly less power than the 4870 and 4850 do on idle.
You can permanently downlclock your cpu, but i don't know how to do that. You can also lower the core voltage of your cpu, but if you do not lower the multiplier/fsb along with it, it will probably not be very stable.