E.g. break the edges sometimes, to make the geometry less blatant. If you have more complex geometry you can do more clever stuff than what I'm doing here. When you have softer edges you can start doing other things of course, like blending them together with gradients. In the image posted by someone above, no edge highlights are needed because there's contrast with the baked in shading anyways. Gradients on flat faces can make things seem buckled and curved (unless it's baked in ambient shadows for armpits and such) so sometimes it's better to just go flat if the readability of the face angle is important.
I'm probably being a bit to rectangular here, the panel lines could very well fight the blockyness instead (with angles and curves). The crux is to be able to do that while also following up on the form of the geometry.
On the Megaman Legends Reaverbots they did 45 degree bevels on very square stuff just with a baked in lightsource. See body and head of the Arukoitan. It's a little harsh when the bevels try to soften a 90 degree angle though, but this was a PS1 game. What I mean is that in some cases it may look nice to add geometry just with baked in shading/light.http://www.legends-station.com/?page=motb/rvr-arukoitan
They used a pretty flat style and softened edges rather than highlighting them. See the circumference of the 'Horokko'.http://www.legends-station.com/?page=mml2/rvr-horokko
The Kurobun has no edges around the legshttp://www.legends-station.com/?page=mml1/rvr-karubun
Edit: I'm not really critiquing Ptiong here, these are just things I've been thinking about and wanted to write down. If you look at the SupCom UEF
stuff, they're doing a lot of beveled boxes inside the polygons, like on my red cube.