Psychology

Speaking is about forming a connection with the audience, it is about that aura which lets you create a lasting impression on their minds. Few things that can help in motivational speaking or any kind for that matter.
1. Getting the pulse of audience.
2. Being precisely up to the mark with their needs.
3. Understanding different changing paradigms which are letting people into de motivation. For ex - with the advent of social media the meaning and reasons of de motivation changed.
4. Effective usage of models, their evolution and case studies.
5. Better grasp on scenarios, making the person affable amongst audience. Above things will come to you by studying psychology as a subject. So, it helps. Ultimately, thing is how you use a thing rather than having just having it ;) to your best advantage.