Most illustrators today have a computer. If they blog, send files to publishers, have an email account, or just want to check the internet news there is often a computer at their disposal. Great news!
Your friendly computer can become an illustration file cabinet. Before we had this tool many of us kept pages and pages of magazine clippings as references. Neatly filed of course. Yet our sketchbooks numbered in the dozens. Finding any particular sketch was a challenge unless they had been filed in the same way as the magazine clippings. More often they would have been stacked or stored in some other way.
Here's a handy link to creating a Digital File Cabinetwhere you can always find what you are looking for.