The trick is to take on more work than you can handle, with a looming deadline and work yourself to death. Hmm... I just made myself very sad.
But the more helpful answer is that I did all of these Red Sonja cards in stages. I penciled them all, then inked, then inkwashes, then color(doing all of the red hair in one pass, sepia tones in another.). This was a bit like a one-man assembly line. Not the best way to work, but I had to do 110 sketches for this set.
But I've been doing these kinds of large sketchcard gigs for a few years and it's made me a bit faster. I'm naturally pretty slow, particularly with sequentials, but the more you do it, the faster it becomes. Hope that helps.