Being over-prepared, centered, experienced, and the positive feedback from my evaluations.
I also look up the professional journals a specific group probably reads and incorporate that into my presentation.
A hint that came from Phil Sorentino, a professional speaker, said to always get introduced before you speak.
If you tell them your credentials, it looks like bragging, but if someone else does, it gives you credibility and makes your audience ready to learn.
When I first tried that, I got the best results I'd ever had from that particular kind of class. Now I ask the person who hired me to do that--I email them the introduction and also bring it with me in case they forgot it.
Getting quoted in the media as an expert certainly helps! But when I got the Career Achievement Award from my college at Ohio State, that was definitely the thing that really made me feel like at expert!
Bonnie James Professional speaker and trainer, owner of Advanced Reading Concepts Speed Reading Plus courses nationwide