Make sure you are adaptable when speaking to different kinds of groups. There is a vast difference between speaking in a community senior citizen center and a gymnasium full of teens thrilled to be out of the classroom. The key to adaptability is anticipation. If you deliver a speech the same way twice, you are not communicating but rather, reciting. Talk to me.
Communication is the goal in effective speaking.
Just be open to making adjustments as needed. An extra word here about teens. Occasionally teens enjoy asking questions or displaying poor etiquette for the mere purpose of shock value. Humor is welcome, but not at the expense of those waiting for the gift of life or at the expense of those whose family supported their loved one's decision of donation. Trust me, it will work like a charm.