Know you're talking about. That alone will make the talk more natural, as you'll be able to adapt to your audience due to having knowledge outside of just your research.

This was the biggest change between the presentations I had to give in high school and the ones in college and above. In high school, the topic was usually given to us, whereas in college and later, I got to choose the topic I was most knowledgeable about. The latter meant I was more passionate, more able to answer audience questions and more able to tailor the delivery to the audience.

One piece of advice I heard was that if you absolutely have to talk about a topic you're not familiar with, talk about being a beginner in that topic. Talk about your perspective as an outsider learning that topic in the first place.