I like using Pigma Micron .005 black pens for making panel lines. They're basically a disposable technical pen, I've seen them in most art supply stores and Hobby Lobby/Micheal's. I like to apply the panel lines on an intermediate coat of paint...apply a few color coats, let dry, apply the panel lines w/the Pigma Micron pen, let dry, and apply a few more light color coats. This way the panel lines are ultra-clean, and a darker shade of the color. It's also quite easy to do-less messy than using paint, although the tutorial on italianhorses.net is my second-favorite method of applying panel lines.
An assortment of pigment liners/disposable technical pens that can be used to detail models as well.
.005 is the smallest shown, also shown is .01 and .02
This picture is huge, the line looks a bit ragged, but you don't see that on the model which is much smaller than the picture.