I use black lining and "trompe l'oeil" painting. I call it "painting details where is no details".
I constantly use these technics for soft plastic 20mm figures, and miniature football 32mm figures. Here's some examples :

On this figure, I coated the weapon with stuff to make it bulkier and sturdier. The weapon itself lost almost all details in the process. It's why I bring painted details on it, using strong contrasts, and black lining. Simple as that. It can look a bit rough on pic, but on the tabletop (and even in the display case) it "works" pretty well.
On these figures, I painted muscles, faces, chinstrapes, etc...on figures with smooth surface and almost no detail.

It's a very fun technic !