Even though our preference may be to have fake conversations in our head versus the real conversation, planning what you are going to say in advance is actually an excellent skill for several reasons. First, it gives us time to consider who the audience may be and possible topics of interest . Second, it helps prevent the brain freeze that can come from being put on the spot. This may still happen but you can still pull out your arsenal of anecdotes once you have warmed to the room. Having little stories about ourselves humanizes us to others even if we feel like they may not be important or silly. Finally, having things to say in advance makes us appear more confident and prepares us mentally for the higher level of energy needed to interact with a lot of people.
One of my tricks is to have several stories filed away in my head that center around my kids or my pet. If I know who I am meeting with, I select the stories I feel will be most entertaining. They are never long, just bite sized glimpses into my life that satisfy the crowd and leave plenty of space for the Extroverts to take over the spotlight. If I am meeting with total strangers, I let them speak first and monitor the conversation like a game of double dutch and jump in when I can feel the ropes open up. Sometimes I get tripped up and that's ok! Laugh at yourself, take a breath and start over.
One final note about preparing in advance: pauses. Pauses are good. Breaks in conversation when you run out of or forget things to say are normal and the silence seems much longer to us than to others. If this happens and you feel yourself starting to squirm, look for something to distract attention so you can get your thoughts together. Something simple such as as "wow, the food is great here" or "it's a little chilly/hot in here isn't it?" Whatever you feel most natural saying will help you get comfortable faster. If all else fails, you can always excuse yourself to make a phone call or use the restroom.
Give yourself the time and permission to think about comfortable stories you can store in your mind and allow yourself to pause or even freeze. Make sure you are well rested and have had enough alone time to recharge before you enter a conversation. Introverts have a lot to say and it's not always out loud.