Ask HN: How to think on the spot and persuade?

One of the biggest flaws I have is not being able to think and argue on the spot. I suspect it has to do with how grown I've become to Internet forums, which gave me the freedom to go back and edit what I wrote.

Often times when I get into a heated debate, I get flustered from being in the moment. But then I'll usually think of a really good counter-argument like thirty minutes later. Is there any sort of training or maybe any books that I could look into to help me get over this?

I feel this is a very fundamental skill to have anywhere. Thanks!