Basically, this is simply spraypainting a large number of horses at once in black, brown and white. When I am ready to paint cavalry I give the riders a quick wash of brown, do the flesh, glue them to the horses and, because they now look half done (or, at least they do to me!) I find it much easier to motivate myself to add the finishing touches. For light cavalry I will rebase them to convince myself they're even closer to completion. It remains to be seen how well this will work, but the theory seems good so far, and it has worked on a smaller scale of production!
The horses (above), block painted and awaiting riders
Another big timesaver is the old magic wash. I use it in two ways. First, after priming the figures in white or gray I give them a brown wash using Klear and smidgens of the two paints below. I also use a bit of water with this to reduce the sheen which the Klear gives. This wash gets into the cracks and gives a bit of depth to the figure. I then paint in the areas of the figure, leaving the wash to show through in places. This allows me to add depth to the figure without having to apply a darker shade for every single colour I apply to the figure. I then add highlights to some things (flesh, tunics, cloaks, etc) and finish off with another wash of clear Klear, this time to provide a protective coat. I sometimes add just a touch of black or brown to this to help re-define areas of the figure that got a bit messy during the painting stages. I usually give two coats of this. Once dry it's time to give them a blast of matt varnish, and I can move onto basing.