The first is to have a talk with your players at the start of the game, asking if they have any "no-go" areas or subjects you should keep in mind. Perhaps start by telling them yours. For instance I would start this off by saying, "Ok, so before we start. Please let me know if there is anything that you do not want to deal with in this game. For myself, I do not handle animal abuse, child abuse or sexual assault well. This does not mean we can't work with them at all, but it has to be done sensitively and very, very, very offscreen." Or if I'm having a more "tender" day: "I absolutely cannot handle sexual assault and animal or child abuse. Those are not areas I am willing to engage with even obliquely." Once you have everyone's, and please write them down so you remember, thank people for telling you, and ask everyone to respect those boundaries.
This is super important to me, because I run a lot of really dark games. I run tons of Monsterhearts, usually set in other, more adult, locations because as I've learned as an adult, many people never get past that high school mindset, high school was a nightmare hellscape for me, and I feel better if the horrible things being done are being done to adult characters.
And I have definitely run games with a fairly high sexual assault content. But only with players I trusted, in a location where if I had to tap out or fade something to black, I knew people had my back and I could do that gracefully.
But I am very firm about people telling me what will bother them. And sometimes if it's a convention game that's set ahead of time, I will ask the players to email me their limits so that I can announce the no-go topics at game, but no one has to say them out loud themselves.
In some communities this discussion is referred to as setting "Lines and Veils." It also has an analog in the BDSM community, where consent and communication are really important (yes, they are, or you are committing sexual assault). I'd rather stick with the terms I'm more familiar with, "Limits and Boundaries." In both BDSM and gaming parlance, a Boundary is something that can be pushed at occasionally and with sensitivity. If you, as GM, are not capable of that sensitivity, then consider Boundaries to be Limits.
A Limit is something that is absolutely NO. For the first several years I gave BDSM seminars and panels, we would invariably get one person who would interrupt this discussion to ask, "But isn't it possible to a sub(missive) without limits?"
To which the Geek Husband What Rules said, "So, you're ok with it if I break your arm, rub poop in your hair and toss you out a window?"
"All right. There's your Limit. No poop, no arm-breaking, no defenestration. Three Limits right there."*
Everyone has areas they don't want to engage with while having a good time, no matter how trauma-free their life is. And those folks are usually the ones who hit the most landmines in game. Because they don't think about things that upset them, like that horror movie an older sibling made them watch when they were five, and the first time a bad guy threatens to pull out their teeth or fingernails to get the information they want, they will freak the hell out.
Which brings me to my next point
For someone with PTSD (and for many people without PTSD) triggers are bound to happen eventually. You'll be going blithely along with your life, congratulating yourself on dealing with your trauma in a mature and adult fashion, and then someone walks by wearing the wrong cologne/perfume, you see a building, you hear a song, and you're cowering in a corner sobbing, or however you respond. So, it's best to have a plan. And if you're going to spend a lot of time with other people (gaming group), maybe let them in on this plan.
And if you're one of the folks who think they don't have triggers and find out the hard way, that, whoa, yes they do, hopefully you can pick up some tips and tricks for yourself and for anyone else who might be triggered in your presence.
The first step is to be aware of your players.
If someone's entire demeanor has changed, if they came into the game bubbly and happy, and are now tense and jumpy, or quiet and withdrawn and look like they might cry, pay attention.
The second step is to check in.
You don't have to do this in big obvious ways. Call a bathroom break, and give everyone five or ten minutes to get to the restroom or grab snacks. If you can, pull the player you're worried about aside and ask if they're ok, and what they need. This also gives players who do not want to talk about this with you, an option for a graceful exit.
Hopefully, if you did your job at the beginning, they will feel comfortable telling you either what they need to continue playing, or that there is no salvaging this for them and they need to go. DO NOT TAKE THIS PERSONALLY. Shit happens. Landmines are inevitable, even when you take the best precautions you can.
The third step is to Listen.
Listen to what that person tells you. If they tell you they want you to keep going and they'll sort their own shit out, do that. If they tell you they want to keep going, but they need a certain resolution (a happy ending for their character, the bad guy to meet a messy end), do that. If they tell you they really just want it to be hand-waved, and faded to black so that you pick up past the incident without discussing it further, do that. And if they ask if you can back it up a scene, and not have that have happened, do that.
You heard me. RetCon that shit.
Fuck your artistic vision, this is someone's emotional well-being we're talking about.
There are a number of different ways other players can handle a player who has been triggered, and they are probably going to take their cues from the GM. So be mindful of what you say and how.
I know that this all sounds scary and heavy. It does, and to an extent it is. BUT please do not let it scare you. Some GMs and gaming groups go ages without having to deal with this stuff. Some, because of who they game with or the subject matter of those games, deal with it more often. And when players feel safe, and know that they can tap out if they need/want to, in my experience, it makes them feel less threatened, less likely to BE triggered and more likely to get through triggers with minimal disruption to story and game flow when it does happen.
If Ogre's running a game for me, I will give him permission to run roughshod over my known triggers, because with him at the wheel (and with certain other players) I feel like I can safely explore those things in ways that might be beneficial to me.
Some people do not ever want to do that, and that is ok, too.
Dealing with your triggers is hard. Seriously, I have one trigger that is really, really hard to avoid particularly in a work environment, and so I sometimes have to excuse myself to go the restroom and cry and hyperventilate for a bit before I can cope. Then I get home and immediately go to bed because it is exhausting. Emotions are hard work, and our culture is not awesome at teaching us how to deal with them. So I do not blame people who do not wish to engage with their triggers, one little bit. It's hard and sucky, and isolating.
The thing to remember here is that not everyone processes triggers, trauma and anxiety the same way. What works for one person, does not necessarily work for another. So, that asking step is very important.
Like I said I know it sounds scary, but it really isn't. Mostly, it just involves taking time out to check in with people, making sure everyone's ok, and going from there.
*The GHWR asked me to assure you that he is very not into those things. Those kinks are for demonstration purposes only.