For any effect that would normally be save or die, a failed save instead reduces you to negative 1d8 HP and Dying status (basically the house rule Xor referred to in the original post, but slightly less nasty). Your friends have to reach you quickly in order for you to live.
I have death occur at negative HP equal to your Con ability. I also have "disabled" apply from 0 to negative your con modifier for more dramatic effect.

So somebody with a 14 Con is disabled at 0 to -2 HP, unconscious and dying at -3 to -14 and dead at -15 or more. It's not a huge difference, but it helps, especially at lower levels.