The book "Twelve hours of sleep by twelve weeks old" outlines a plan that I just started today. I'll let you know whether it works. Basically you feed the babies 4 times every 4 hours during the day (8am, 12pm, 4pm, and 8pm, for example). After the 1st feeding there is a 2 hour nap and after the 2nd feeding there is a 1 hour nap. Between the 3rd and 4th feeding they are kept awake. During the night, between 8pm and 8am, you feed them (just a little bit) when they wake up and cry and can't get back to sleep. You gradually (every 4 days) decrease the amount you feed during the night by 1/2 oz.

I've already been feeding them just 2 oz when they wake up at night, and they go back to sleep for 4-5 hours after just that little bit.