I like to use the back of a #11 X-Acto knife also and like Steve says, I make sure that the tip is slightly broken off, for some reason it works better this way. I like scribing my lines pretty deep and using Testor's Metallizer paints. Since these paints consistency is very watery, capillary action helps a lot when using this method. I will do this on the bare plastic first and also after my primer coats, after this, everything pretty much falls into place.
Here is an ongoing project of mine, the kit plastic is yellow, this is not the actual color.
Hosted on Fotki
Hosted on Fotki
These are the results after painting and completion....
Hosted on Fotki
Hosted on Fotki
Hosted on Fotki
What I like about this method is that you can notice how deep the panel lines actually are, in my humble opinion, it looks as if you can actually open the doors or trunks on the model. You can still fill the lines at the end if need be even after going through the original process.