There probably isn't a method that will either kill you or cause no damage. Every method has its risk of damage if you fail. It really depends on when the intervention happens. For example, if you try hanging and the ligature either snaps while you're still conscious or you back out, you should recover. But if you get found later on, maybe even when you're clinically dead and revived, there is more of a risk of being a vegetable. People also say there won't be permanent damage if you survive SN, but it's hard to say. It's unpredictable whether you'll make a full recovery from a failed attempt or not. Two people could make the same attempt, one could make a full recovery, the other might have permanent damage. There are too many variables to say for sure.
The best way to avoid permanent damage is only doing the act when you are fully committed to it (if you're ambivalent about ctb at that moment, don't do it) and also look at reasons for why you might fail, and then prepare in advance. For example, is the concern being found in time? Then book a motel or hotel room if you can, go to an isolated area, or ctb when you know you'll be home alone for a long time. As for the methods itself, it varies. If you want to do SN, make sure you follow all instructions (fasting, antiemetics, acid reducer, etc). Are you jumping from a height? Make sure that the height is high enough that you won't survive jumping off (higher than the golden gate bridge, it's rare, but people have survived jumping off of it). If you have any methods in mind and want help with making it as fail-proof as possible, feel free to ask away. :) Hope this helped.
