i know the feeling! last year i was in AP seminar, which is a lot of presentations and it just so happened that the words related to the topic i was researching were all words that i could barely get out :P the best i can say is keep practicing what you are going to say so that you can figure out what sort of techniques you can employ. like when i prepare for presentations by practicing, sometimes i realize that i need to reword some part of the script so i don't stutter, or i need to breathe a certain way, etc. practice practice! rooting you on! even if you do stutter severely at the expo, you'll do great!!