A note about static
Static electricity is the buildup of electrical potential. When that little blue spark jumps from your fingertip to a nearby object, it's because that object has a lower electrical potential than you do at that given moment. Although it might pack several thousand volts, static electricity's amperage is so low that the "shock" you feel is no more than a tickle. However, that tickle carries more than enough amperage to harm your motherboard and almost anything plugged into it.
The electrical cord connected to your computer provides grounding, but only when it's plugged into an outlet that's properly grounded. While you can safely leave an AT case plugged in while you work inside, most ATX systems have a feature that supplies power to the motherboard even when it's turned off. You could end up frying one of your vital components if you leave it plugged in. Unplug the power connectors from the motherboard itself, but leave the cord to the power supply plugged in. This way, touching the bare metal of your case will diffuse any built-up static, and anything going into your motherboard will be safe from potential arcs.
If you don't want to be bothered with plugging and unplugging the power to the motherboard (which can be a real hassle when you're troubleshooting a hardware installation), at least get rid of the higher electrical potential by frequently touching a metal chair or table--preferably not the one the computer's sitting on. If you want to be extra cautious, touch a nearby cold water pipe to ground yourself.
Your best bet is to slap on a grounding strap, which is typically a bare, braided wire that wraps around your wrist and connects to the center screw of an electrical outlet. You can find one at most electronics stores.
