Neil deGrasse Tyson is one of the best at this. He says his secret is to be 10x more prepared than he needs to be before every interview. 90% of everything he says was written down. You gauge your audience, what they could ask, and anticipate all the things they could say.

You have to write it to make it seem clearer. Thoughts are a lot like arithmetic; you can do 385 x 445 easily on paper, but not in ypur head without practicing the right technique. Heck, even small things like 629527 + 111111 + 111111 is really hard in real time.