Hi ..for me, it relied on doing the prep work & on a practical level the 2 things that helped the most (in the moment) were relaxation & being my own detective (& probably exposure therapy too & most importantly). At the end of the day though (as i'm sure you're aware), dealing with the underlying issues is what eventually put an end to it for me. But this takes time & in the meantime, these are what helped me. (Side note: key for me was taking the time to find a helpful strategy & to develope it, one strategy at a time. Once i had one down, then move on to learning a new one. In the end i had a toolkit full of strategies. Didn't matter if they helped the way they were suppose to or not, i just knew that if i did 'this' it would help me in 'that' way & 'this' helps in another way etc. But i knew what they did for me. The trick was to use them like recipes & stack strategies relevant to what i was dealing with at the time. Might seem obvious but took me awhile to figure out.)
Took me the best part of 20 years to find a relaxation technique that actually worked for me. By worked i mean it actually got me to the place of relaxed. I started out doing this for 10 mins (would take the 10 mins to hit relaxed, not 10 mins of relaxation), morning & night, for a week or two. Then i tried to keep it up once a day (but never would - sporadic at best). I have to say that this never helped in the slightest with minimalsing my PTSD symptoms overall or ever helped me to become more relaxed generally. But what it did do was create a reference point of my body being relaxed that i could recall on in the moments i needed to. Something like muscle memory, where the more i practiced it, the more reinforced it became & the more influence it would have when recalled.
The other thing i would do was a technique that was shown to me where i would be my own detective, actively, consciencly & deliberately be looking for all the evidence around me & pointing it out to myself, showing not only that i am actually safe but just how much so. This is of course assuming that you are aware that your hypervigilance is an overreaction to the current situation at hand. (If not, there is probably some other work needed to be done instead). By commencing my relaxation breathing & thinking back to doing my relaxation technique, i would be able to recall a relaxed state (even if it was only momentarily) & that would give me enough sense of mind to be able to do thIs work. If done well enough, i had succeeded in creating a sense of inner calm or peace, knowing that i was in fact safe & ok, even though it didn't 'feel' like it at all. Enough so that i could think rationally & be functional until things settled down.
I can elaborate on the type of exposure therapy that i did that helped me if you are interested, but if my hypervigilance was bad enough & was being triggered by something or someone in particular, even though this isn't exposure therapy, i would use the core principle of exposure therapy in these situations. I would choose to remain in the situation, no matter how bad the sypmtoms got, i would use the previous two strategies & stack them with as many other strategies as needed to clam everything down, even just slightly & even if just for a nano second & then decide to leave. And not before. This, together with exposure therapy took situations for me where i wanted to completely 'toss my biscuits' gradually, bit by bit, down to no more than feeling a little nervous ..to end up actually olooking forward to & encouraging others to go as well (of course having dealt with the underlying issues as well along the way).
I would suggest that physically retraining your body & teaching it to respond more appropriately to the environment you are currently in, while only half of the equation to overcoming PTSD, is really important work. Good question.
Take care. ??