I really don't think that someone can really prepare or "train" themselves mentally to deal with truly traumatic experiences.
Either you do or you don't. It's really on the individual, and you won't know until it happens.
That being said, I completely think that people can build up their mental health to deal with common inconveniences or fears.
I remember a few years back my brother was involved in a VERY minor auto accident that I happened to be in the car for. He LOST it. Complete breakdown. I have no idea why, both parties had insurance, the cops issued him a simple "failure to yield" citation (no jail time or revokation of driving privledges), and the damage was really very minor with no physical damage of anyone to report. It bugged him for weeks to the point where he was not sleeping well and not eating properly. Eventually he returned to normal, but it was very bizarre.
That is a perfect example of something that he (or anyone) could mentally train themself to not completely flip out about.
As far as myself? My wife just gave birth two weeks ago and our daughter was concieved via c-section. I had absolutely no problem looking at my wife cut open. Not because I'm some weird freak that enjoys seeing people with their guts sticking out of their body... but because I mentally prepared for what was going to happen and when I looked at it, I knew what I was seeing, I allowed my brain to process it, and I didn't allow myself to get freaked out. I knew what the doctors were doing, I knew they had to do it to deliver our child and I also knew that if I lost it, my wife would probably lose it too. I was defintely able to prepare for that.
Now, if I came home from work and my wife was cut open in the same way and bleeding to death? I don't think that there's any amount of mental preperation that I could do to prevent myself from being stressed out for a LONG time. Maybe I could prepare myself to not panic in the moment, but something like that is one of those things that would stick with anyone for awhile.
So, I guess what I'm saying is that I agree that mental preperation is a useful/productive thing. However, preventing PTSD from a truly traumatic event? I don't know if that's really possible. I guess it can't hurt to try, but if someone, say, loses their entire family in front of them in a traumatic way (or suffers any other really unexpected traumatic occurance) I could completely understand why it would linger with them for a long time.