The worst part about hanging drywall was cutting out around electrical outlets. Measuring only got me close -- there was always extra trimming (and dust) and the hole usually wound up slightly oversized.
Then I made a template that lines the hole up perfectly over the outlet. I started with a rectangular scrap of 3/4"-thick plywood and traced the outline of an electrical box on one side. Then I cut out the center area so the template would easily slip over the outlet box.
To use the template, I put some double-faced carpet tape on one side of the template and slip it over the wall-mounted outlet with the tape side facing out. Then I position the sheet of wallboard on the wall and press it against the template. With the tape holding the template to the back of the wallboard, I lower the wallboard down.
To cut the hole, I trace around the template with a pencil and remove the template. Then I make the cut with a utility knife or keyhole saw. You could also leave the template in place and use a router or laminate trimmer with a pattern-cutting bit, but it raises more dust.