I'm an editor and I agree with the previous posts. Printing hard copies is a terrible waste of paper, but it's an excellent way to identify mistakes that we miss on our PCs. Taking time away from the piece also works.
In addition, I eliminate all distractions. I used to listen to music and reread my writing at the same time. Not anymore! I unplug the phone or let it go to voice mail, and will only proof important documents when I'm feeling alert. Maybe there's a particular time of day when you are more alert than others.
Also, often I magnify my screen up to 150 or 175 percent. It's downright embarrassing, but helps me to catch a lot of little errors. And I will spell-check the document in several places. Maybe I will check it once in my e-mail reader and again in Word. Sometimes things slip past me in Word that are picked up elsewhere.
Good luck. Sigrid
PS Don't accept all of the suggestions by the spell-check in Word. It can often be wrong.