The short talk (<= 20 minutes) is a fixture at most scientific conferences. Assuming that you have an overhead projector, this note focuses on how to give a good short talk. I think that if you can give good short talk then you can probably give a good 50-minute presentation because the additional time permits a certain flexibility. For example, in a 50-minute talk I think that the speaker can risk losing the majority of listeners for 10 minutes while a technical aspect is embellished for the 'experts'. In contrast, the short talk requires a more sustained level of clarity if it is to be successful.
Van Loan, Charles. Cornell University (2000). Presentations>Advice