Over the years, I have tried various ways to mark and identify circuits as I was terminating panels. Usually by the time I was finished and ready to write the panel directory, it was hard to tell which wire was with what circuit. Now what I do is pull off the outer sheath of a scrap piece of nonmetallic sheathed metal cable, cut it to the desired length, and write the circuit information on it with a felt-tipped pen. I then slip the marked sheath over the conductor as I am terminating the conductors in the panel. I leave the markers in place, so that in the future, if the panel directory is ever lost, all you have to do is remove the panel cover and you will be able to identify all the circuits. Using sheathing from different sizes of cable, such as 12/3 or 10/3, you can identify a large range of conductors. You end up with a neat, professional-looking installation-the goal of every electrician. I have even heard inspectors comment on the practicality of this method. Best of all, it's free!