1. Watch a couple of high-rated TED talks and model your presentation (obviously not content) off of them.
2. Limit yourself to 15 minutes. (edited to add: don't necessarily use 15 minutes, but it's a good benchmark for attention spans. Make it long enough to give them the honor, respect, and dignity they deserve and not one second longer)
3. Make it personal to the graduates with a focus on their next assignment.
4. Include a well-told story with a situation that each graduate could picture themselves in (and later tell their parents "that could be me in a few months")
5. If you find yourself wandering, stop talking, pause, thank them and their families for their choice to serve with all the associated sacrifice, and sit down.