I do it all the time. I'm a funnier, wittier, more fluent and perceptive person in my head than when it's time to actually talk though
. Sometimes it helps me after an argument, I perform way better in round two and beyond, but otherwise I feel bad for dwelling in the past too much. It's entertaining though.
For future conversations, that happens sometimes as well, but I find it more efficient to just make a mental list of the things I want to accomplish or talk in the next conversation, and whenever there is an awkward pause go to the next topic. I can only small talk with a limited set of people, so it's pretty helpful.