Dull black crayons or, failing that......lipstick.

Lipstick?!? Sounds crazy at first, but when I think about it....

Good thinking. Use some red lipstick, and let us know how it works
out in the field ;-)

In all seriousness, you want something that draws *very* thin lines.
I originally used the sharpie fine points, and found the lines
obscured what a few stars. The extra fine sharpies are perfect. Give
'em a try on one chart, and see what you think.

Chris Nicholl