The best kind of pencil for this use is any 'automatic' pencil that takes 0.7 mm or 0.5mm lead refills. (I'm not sure what the equivalent sizes would be in the US). You press the end of the pencil to advance another little section of lead. The reason this is better than an ordinary pencil is that the thickness of the line stays much more consistant that an ordinary wooden pencil with it's much wider lead, and having a narrow parallel sided piece of lead allows you to draw inside the bottom of the panel line, rather than leaving the graphite deposited along the top edges.
The best grade of lead to use is the medium to soft (2B or 3B should be fine) harder grades will tend to scratch the paintwork rather that leave a nice dark line.
Hope this is of help.
Phil