I spend a lot of time thinking about important conversations beforehand, and the directions they could take. I'll pace around the house for half an hour talking to myself, practicing speaking. This has proven helpful in my job, which involves a lot of meetings and conversations.

To the original commenter's point, you don't want to memorize your answers, you just want to explore these conversational avenues and test-drive what you might say, seeing how it sounds, what positions are more defensible, what topics to avoid. This is the skeleton of the conversation you'd like to achieve, the "plan".

Of course no plan survives contact with the enemy. When conversation gets spontaneous / goes off the rails, I try to detach and control my emotions. Sit for a few seconds, consider what you're about to say before speaking, and be straightforward and honest.