Much to my chagrin I found out (through experience and research after the experience) that an opiate overdose by itself isn't that likely to kill you.
I would recommend drinking heavily beforehand and taking any other sedatives you can get your hands on, such as benzos.
Apparently it's the combination of CNS suppressants that typically results in death.
Wish I'd fucking known that before jamming 800+mg of heroin into me…