The survival instinct is always going to be around, there is no getting rid of it permanently, but one can come close or at least overcome it temporarily just enough to ctb. For me, over the years, I've been just watching lots of violent videos, violent films, gory scenes on the Internet. This desensitizes me to death when watching people die and what not (e.g. Budd Dwyer committing suicide on TV, a prisoner in a police station suiciding with a concealed handgun, and more). Then after that, when I get around to getting my method (firearm), I would practice dryfiring it as well as aiming the barrel at where I can hit my brainstem (in the mouth, at an angle), and then letting myself become accustomed to it. There is more to it, but these are the basics of overcoming and trying to override the survival instinct.
Alcohol and drugs can help, but being too drunk or heavily drugged could compromise one's attempt, resulting in a higher chance of failure. Also, not everyone has access to drugs and don't always have alcohol readily available.
