Its not just what you say, its also how you deliver your presentation and interact with your audience.
Harvard Speech & Parliamentary Debate Society: The team's web page
Public Speaking Practicum: Course website for Expos 40, a course on public speaking. The website includes public speaking resources and readings.
Public Speaking & Negotiation Club: A Harvard Business School club devoted to personal development of public speaking and leadership skills.
Improve Your Pubic Speaking: Harvard Business Review blog post on improving public speaking
Need Help Giving a Speech? : Resources to help prepare for giving a speech
How common is it to write out a script for a talk?: Academia StackExchange question on how to prepare for an academic talk.