What to do in the ten minutes before, and what to leave alone
The nerves before you speak are not a malfunction. They are your body getting ready, and the aim is not to feel nothing. It is to keep the readiness from tipping into a shake.
That distinction matters, because most of the usual advice aims at the wrong target. Trying to feel calm when you are not is a losing game, and the effort of it tends to make things louder.
Telling yourself to calm down asks you to feel something you do not feel. You then check whether it worked, find that it has not, and now you are monitoring yourself as well as the room. That is more to carry, not less.
The workable target is steadier, not serene. A racing heart and a clear head can sit in the same body. You are not trying to remove the first, only to keep it from driving.
If the nerves arrive all at once, the physiological sigh is the fastest thing available: two inhales through the nose, the second a short top-up, then one long exhale through the mouth. One or two of those is usually enough to take the top off.
It works because the second small inhale reopens the tiny air sacs a long exhale has just emptied, so the exhale that follows can be longer and slower than the one before it.
Caffeine in the hour before adds to a heart rate that is already up, and it does not add anything you need. If you are going to have one, have it early and not on the way in.
Practising the whole thing from the top, over and over, mostly rehearses the anxious version. One run through the opening is enough. The middle looks after itself once you have started.
The nerves will not vanish, and they do not need to. What changes is what you do with the first ten minutes, and that part you can practise in advance.