If you have time before your impromptu speech, create an outline of the major themes or points and commit it to memory with a memorization trick, like an acronym. Don't try to remember the entire speech in detail like this; just remember the order of important points. Talk about things you are familiar with. If you suddenly lose your train of thought or draw a complete blank, there are a few you can do to keep from panicking. 1. Pretend like you're pausing on purpose. Walk back and forth slowly, as if you're letting your last point sink in. You can appeal to the audience by saying, " How would YOU feel if this happened to you?" 2. There is always a joker or popular person who will stand out in the crowd. Stare at someone like this and try to draw a response from him or her while you think. Say, " Some people may not find this topic serious, but I do. How do you feel about the environment/pollution/nuclear power etc?" 3. If you need more time to think, you may want to ask the audience a question. Have a few prepared ahead, like "Do you have any questions," or "Can everyone hear me okay?" 4. If you still can't remember what to say, make up a reason to pause the speech. You can say, "I'm sorry, but my throat is very dry. Can I please get a glass of water?" Someone will go to get you a drink, and you will have time to think of two or three points to talk about.

anticipate what possible topic your going to deliver and from there, you need to research. try this link. it might help. http://www.wikihow.com/Give-a-Great-Impromptu-Speech :)