From what I remember, Manny actually uses a mechanical pencil on his panel lines. You'd have to find a really fine pencil lead for 1/72.
Another simple way to do it is to use ProModeller Wash. It a watery solution that you brush on, let dry and then wipe off. It stays behind in the panel lines, giving you the look you're after. It comes in both dark and light wash, depending on your paint scheme. If your paint is a dark color, use the light colored wash. If it's a lighter color, use the dark wash. Couldn't be a simpler product to use.
Now, I don't work for these guys, I get nothing from them, it's just the simplest stuff I've found that actually works. I use it all the time.
You can get it here http://www.promodeller.com/
-Fred