+1 on everything dancingsinging said. Or, if I'm in a situation like a meeting, I will say something like, "I'm sorry I'm not expressing myself well. Y'all please go on for a minute while I try to figure out how to say this." Then I take a minute to write, outline my thoughts, and try to come up with another way to say what I mean.

Similar to what wild_irises said, I have a cue for myself, but it's physical - I get this feeling in my throat like something is stuck in it, but instead of wanting to swallow, I urgently want to speak it out.

Usually when this happens to me, there's a big emotional component to it. What's going on is that I'm getting a particular button pushed; I am feeling deeply misunderstood in some fundamental way - like the Samuel L. Delany concept of "rupture" - but it's personal, and I feel like I am being mistaken for a kind of person I am not and do not want to be. So as soon as I can, I go do something that lets me zone out (driving and wailing along with beloved songs is excellent for this) and I try to figure out what was pushing my buttons. Then there usually needs to be some combination of talking with a trusted person, chocolate, carbs, wine, and being held. Sometimes it is mostly intellectual, and in those cases I just need the zoning out and possibly some ranting to my partner afterward.