Mostly experience, there is no way to speed it up at least I don't know of a fool prove way to reduce this besides just experience.
Though maybe you could try at random points of the day to just pretend you're in a dream becoming lucid and remember that you want to stay calm and think over what you want to do next. Kinda like preparing a speech in front of a mirror, so when you finally step on the real scene you will not be as nervous.