By continuing to squeeze, you will be able to shape and mold your comet. Also have the students listen to the comet, as it will fizz for a few minutes. The most important thing to remember is that you need to have equal parts of water and dry ice. I have found that a comet using 3 cups of each is a nice size and continues to fizz and give off gas several minutes after removing the comet from the bag. If you do make a comet this large, use a gallon-size bag, rather than a sandwich bag.
Some people also add ammonia, commonly found in comets, to the mixture.
However, this tends to bubble up and make more of a mess, without adding much to the demonstration. Try the Jet Propulsion Laboratory's Teaching Resource Center. Because of the popularity of these tapes, and our office moves, we have developed quite a back log of unprocessed videotape orders. Please be patient, it might take many months to get your tapes. The Jet Propulsion Laboratory is one of NASA's outstanding educational outreach resources.