Tip for Never Losing Your Master Copy

I don't know about you, but I am always losing my master copies!  It seems that I give them to students, leave them in the copy machines never to be found again, or pass them off to a co-teacher not realizing they were the original.  Several years ago a colleague of mine gave me a terrific tip that has saved me tons of time and frustration.  I use a yellow highlighter to make a small m at the top of my master copy.

It is a signal to me that it is my original and when I run it through the copy machine they yellow highlighter does not copy!  Simple and effective.  What time saving tips do you have?

Sean