@WeaponOD: Yeah, practice, really. In particular, drawing lots of things from reference till I had a basic grasp of anatomy. I still fuck it up a lot, though, so I clearly need to practice more!
@Makasu: Why not draw normal armor? :p Boobs are squishy and will easily fit into male armor, and male armor is frequently designed to actually protect you, which is a plus. And as far as brush settings go, I just use hard round brushes. I generally do a rough sketch and then ink over it for a smoother result. I'm also anal-retentive and clean up my lines if they aren't super perfect the first time.