I always prepare in the sense that I know the material that I want to present like the back of my hand. But I never practice. People who actively practice come off sounding like robots, and I think it makes it worse when they forget what they want to say. Its better to have a general idea of the direction you want to go, and let the slides (if you're giving that type of presentation) prompt you. I think it comes off as far more engaging, natural and fluid.
People who actively practice come off sounding like robots
-Good point. There is such a thing as being too scripted or overly prepared. Everyone is different in this regard. For me, the best way to go is to know my details but just use topical bullet points. No script, just a road map.