When for the 1st time you look up the phone number of a person and/or a business take a pen and circle the name of the person and/or business. Then the next time you have to look the number up again it will stick out like a sore thumb. Of course if this is a person and/or business that you will be calling frequently then enter it into your address book. Sure saves a lot of time.
Thing is, if a person used a highlighter when "circling" phone numbers, the the numbers on top and bottom of the one circled won't be crossed off. The companies that make highlighters make them in various colors, so if you are looking through the "yellow pages" you can and should use a color that stands out on yellow paper; yellow highlighter might not do the trick for that page.