I love to work "en Grisaille"!
I'm in between two big workshops and these are precisely the main topics I'm bringing "on tour" these days, so this video is part an anchor for old and new students, a video reference/preview of the workflow and its general mindset, and part me stretching and exercising to be ready for this weekend's event :D
If I have to choose, gun to the head, a single drill to improve the understanding and quality of painting it would definitely be value sketching and grisaille;
it strips the miniature painting process (or any other painting process) of all the fuss and nonsense of the step by step/commercial/GW... way of painting showing with a striking simplicity what's really essential to put on a model.
This doesn't mean that you shouldn't paint directly with colours or use any other approach (I constantly move between methods and techniques) BUT if you understand these idea and you can render something complex in a simple black and white, when you add the informational and emotional power of colors to the mix the result is a million times more powerful!
Or, the other way around, if you can't render something in black and white, well colours will not help you at all to get to the point!
ps: the Retarder trick with Contrast is really that powerful! I'm afraid I didn't emphasise it enough in the YT video, but yeah, it really brings that kind of paints close to perfection ;)
Enjoy!