Shadrach and Hamish have given some very good techniques, some of which I often use when speed painting.
One trick I have grown to rely on is painting figures while they are attached to the spue by the base. I've taken to making all customizations, and fully painting figures before I cut the bases loose.
Another useful trick in speed painting is the application of washes. A red-brown wash will hide a great many errors when you start massing figures into battalions. I like to block in the major colors - jacket, trousers, equipments, flesh - then apply a dark wash to fill in shadows; then, I pick out key details - belts, straps, facing colors, metal, facial hair. The result is pretty good, and the more figure you have the better it looks.
I'm also a firm believer in prioritizing the quality of the paint job by rank. NCO's & company officers get pretty good detailing; senior officers get better detailing; army commanders get the absolute best.
My final tip can be the easiest, or the most tricky, depending on your subject. Identify which figures will be in the center of the formation. Only the front rank and outer files need detailed painting. The figures in the center of the column really only need to be painted well enough to blend. One way to do this is speed paint all of your figures on the spue, then base the figures and pick out the details on the most visible figures.