Step 2: Laptops and Tablets
WiFi can suck up a pretty large amount of power. Bluetooth counts also. Turn off these radios while not in use. Some computers have cellular modems. That also counts for this category. These save a great deal of power for laptops.
Interestingly enough, leaving a disc in your CD/DVD drive may actually decrease battery life. I don't know differences for computers, but if the drive spins up occasionally, it uses up a lot of power. One of my older laptops had drives that spun up every time I accessed My Computer. Better safe than sorry, so take out unused discs when possible.
Run less applications in the background if you are not using them. I notice some applications suck up CPU time even while I don't use them. The more CPU time that is used up, the more power is consumed by the system. NOTICE: Windows Vista has a special power management for the CPU that manages the minimum and maximum power state of a scheme. This determines what the minimum and maximum percent of a CPU stays active in a power scheme.
Defragment your hard drive occasionally to decrease file scatter across your hard drive. It would be easier and it takes less energy to read a book like they already are than if the words were scattered all over the page in no particular order, and you had to move your head around like crazy to make up a sentence. Hard drive fragmentation makes your hard drive need to work like this and it uses more power. I recommend using Auslogics Disk Defrag (free) because personally, I think that the Vista Defrag SUCKS!!!
Set a reasonable standby time so that your computer enters standby after a period of inactivity. Set this longer that other times, say 10 minutes. Standby uses much less power than just turning off the backlight. You can also enter standby manually, so you don't need to wait. It will resume quicker than a cold boot and theoretically saves a bit of power. Be aware that the standby resume can use some power as well. Do not enter standby manually if you plan on using the computer 30 seconds after standby. Shut down your computer fully or hibernate if you are leaving it for more than a few hours.
I will add more information if suggested by commenter or when I get the idea for them.