So I wasn't on here in 2012, but I didn't really see this biological answer...probably because there aren't brain doctors or modern psychiatrists on here, but it's relevant if someone was curious of the underlying factors at play.
The Brain: Real world situations are experiences which affect the brain and thus affect the whole body. The hippocampus serves as the memory center which catalogs the memories with the experiences to reference when similar situations arise. The other basic parts that fit in are the amigdila which houses the adrenal & hormonal responses and instincts and the pre-frontal cortex which houses the reasoning parts of the brain.
When a person experiences an event (verbal, physical, emotional, sexual, spiritual) the initial responses register amigdilically and then as the pre-frontal cortex processes what is happening the hippocampus logs it all away as the whole "experience". When a similar experience happens again the reference is in the biology and thus shoots up with the previous experience responses in the body.
Thus we have "training" and some of the other "mock situations" mentioned in the other answers. This stores the experiences in the hipocampus with associated biological responses in the hopes that any future experience of your body (including brain) will know how to perform in order to best address the situation.
Trauma: Any traumatic experience "including being assaulted" would result in the amigdalic response of adrenaline and other chemicals rushing in and require additional processing in the rational pre-frontal cortex in order for the hippocampus to store the memory with full processed experiences. If this processing doesn't happen it becomes what is know as PTSD (Post Traumatic Stress Disorder). The only way past that is the same as originally mentioned in fully processing in the moments(now memories in the hippocampus) that the trauma occured and bringing the pre-frontal cortex into play. Severe trauma like a bomb blast, sexual abuse, any situation resulting in complete powerlessness and damage being done at the same time results in trauma. The "fight/flight/freeze" mechanisms are coping amigdilic responses which are selected based on previous situations that worked stored in the hippocampus. If none are found it defaults to the closest thing that can be found. The need is still there for the pre-frontal cortex to turn into full cognitive responses, otherwise the pre-frontal cortex literally shuts down and the person starts operating completely on the amigdalic responses being received.
Preparation: Thus the major preparation is basically like survival training for the event. Soldiers are psychologically conditioned for combat experiences often by traumatizing them in training and programming via the above process to have default responses for those traumatic situations thus better preparing them to react appropriately when the need arises for real. The best result is full cognitive reasoning with the pre-frontal cortex during the traumatic event.
Please note this is the very abridged summary and there are multiple topics that have volumes of books written on the subject, but unless your going into the profession anything deeper seems excessive for this blog.