My friend who's plugging away at his 15mm ACW stuff says he has had very good results using the following method.
Black primer on all the minis. Heavy, general highlight with off-white paint using a drybrush. Slight zenethal highlight with white ink using airbrush. Wash with heavily diluted black wash (Not sure if he was using Nuln oil or Army painter dark tone).
Paint figs using Citadel Contrast Paints. One light coat to let shading show through.
From what I've seen, they look pretty good for tabletop standard. Some are a bit too dark because he didn't do enough white drybrushing at the start but those minis end up in the middle of the stands and actually look better there because of the increased shadow effect.