It's a pretty simple method, you're essentially just turning potential energy into kinetic energy by jumping from a height and concentrating that energy on your body, allowing it to damage you (hopefully fatally). Understanding that mechanism, obviously the higher the jump, the more energy and the more likely you are to perish. I'm sure there are statistics on the correlation between height and survival rates, I would recommend looking into that. I seem to remember reading that anything over 12 feet is relatively reliable, but don't quote me on that.
Only other thing I could even possibly think of is taking a bunch of aspirin 40-60 minutes prior to jumping, making it harder for your body to form blood clots and stop bleeding. I feel like I wouldn't even want this, though. Only time this would make a difference is if the impact didn't instantly kill you, in which case you would likely be in such pain you would just want it to stop and I feel like you wouldn't even care about CTB anymore out of pure instinct.
Otherwise maybe something for anxiety/worry? Some xanax or a few shots maybe?