What's the best way to break mono finishes?

I would say post shading or dot filtering.  You don't need to post shade the panel lines per se.  Just break up the color by adding white to the base color and randomly apply it so the paint begins to take on a faded look.  Then add two drops of white and do the same thing a little more sparingly.

Then maybe a very light panel line wash on top of that.

To fade paint schemes on aircraft, I mix up a white tinted mix of the prominent colors on the plane and very lighty airbrush this on horizontal surfaces- wing and tail tops, and the top of fuselage. I carry the fuselage coat around the curve, fading it to nothing as I get to vertical fuselage sides.  This lightening of colors represents UV bleaching of paint on planes exposed to sunlight daily.

Dot filtering, hands down in my experience.

What type of model are you painting and what look are you hoping to achieve?