I'm afraid I don't remember the details now -- but it involves getting your money *exactly* to zero before you're supposed to be able to. Once you figure out the choices that get you close and the choices that make that impossible (i.e. earning interest while talking to your family), it's just a bit of playing around with the numbers and working on the problem backwards until you get there. Once you do, the secret endings all branch off from that point.
I went digging through the code and found an early "if money = $0" flag that tipped me off to this -- never would have figured it out, otherwise.