I would say that I often carry this on the edge state of mind (almost PTSD like mindset) where I assume things will likely to go shit and rehearse what I will do soon after the shitty event or incident has passed. I've even fantasized how I will die (vividly) and really flesh out every detail such as where I will stand/sit, what I will eat before hand, and even imagine myself loading the bullet, pulling the bolt/cocking the pump/rack the slide/pull the charging handle, etc. and even imagine myself slowly squeezing the trigger and then finally, total darkness.
I also keep dwelling into the past, the past injustices, think about all the pro-life spiels that make me angry, get jealous about how most others lead happy, successful lives, etc. Pretty much anything to put my state of mind into a deep depression and state of numbness and hopelessness. I feel that once that impulsive event hits and my mind is in that deep, numb, desperate state, then I would be able to act on impulse/trigger event (but it's not impulsive since I've planned it all out thoroughly).
This may or may not work for others, but it's something that I am doing to help temper my mind and desensitize myself to death.