When I got my Yashica A, I took out the screen to clean it (a bit too properly) and lost the frame lines that were printed on it (oops!).
So, I just took a ruler and a fine point sharpie to add them back.
And on my Burke & James, the grid lines were almost completely gone, so I took a scratchy tool and a ruler to "etch" them back in.
I must say that the sharpie is guaranteed to work perfect. And if you take a paint pen instead, you'll get really permanent results.