Clichés become cliché for a reason, and “Practice makes Perfect” is no different. I grew up playing baseball, and you could always tell the teams that practiced from those who didn’t. My team usually had brutal (to a 10-year-old) 2 hour practices that included such glamorous activities as running laps, playing catch and fielding ground-balls. Over and over and over. And over. But of course we became a better team for it. Activities that first required concentration and effort became second-nature. We gained more endurance, became a stronger, more focused team for it. This is the beauty of practice for a sports team.
I’ve found the same is true for speaking as well. When I first began to give talks, I did not practice. I thought that somehow that made my talks inauthentic. Of course, that’s a real danger of being well-rehearsed. We can become slaves to the practice, let ourselves become locked into the structures and styles we practiced. But that is an abuse of practice. It’s serving the system rather than making the system (the practice) serve us.
Good practice makes for a better talk.
Going over your material, organizing it and understanding its flow is essential to good communication. Some of what you have in your notes will not translate well into an oral delivery. If you use any sort of alliteration or physical examples, working through your presentation a few times will help you to get your flow right, so you don’t end up with dead, 
awkward space in your message. Running through your material three to four times out loud before you deliver it can smooth out your transitions and work out any kinks you have in the structure you have built. I often rearrange whole pieces of my talk after running through it a few times because I can hear a better way to communicate it.
Where do you practice? I usually use my office or living room, though I know that some people try to use the actual space they’ll be in. I’ve also found it very helpful to run through my material in front of other persons (my awesome wife usually hears a sermon I give at least three times) so they can give me constructive feedback – what was confusing, what worked well, etc.
Practice done well frees us to live in the moment of the delivery.
I find that the more I’ve practiced a talk, the more free I feel when I am delivering it. I can respond to my audience more comfortably. I can improvise without being afraid that I’ll lose my place. Because the material has become second nature to me. The message has become a part of who I am. So as a consequence, the hours I put into practicing my delivery help me to become more authentic, not less. And I’m not saying that it’s perfect, but it’s at least a heck of a lot better!