Gill asked,
"What is a good ratio for a black wash so as to accent bolts, angles, etc. on armor?"
I'd advise staying away from a pure black wash, because it tends to look too harsh. Instead, go for a darkened version of the base color. You're really painting on shadows when you add a wash, and if you look at a shadow in real life very rarely is it really black. Then, after you drybrush with a lighter shade of the base coat, you'll have a uniform feeling to the piece rather than a 3D line-drawing feel to it.
--Chris Bucholtz