Practice, as has been mentioned.
Here's one concrete points with respect to practicing:
I like to write exactly what I want to say in the Speaker Notes section of the slide.
This point is a little contentious, because the first thing people usually learn about public speaking is "not to read". I used to subscribe by this. Now, I've gone back to writing really really complete notes and reading for practice.
Before I get a whole community down my throat let me explain. When I was practicing in the past, I kept misstating things or adding 'um' or pausing and would have to retract my words, and then that led to even more awkwardness. So I thought, "Well, I'm just going to write down exactly what I want to say then" and not have to try to find the awesome phrasing by memory. So I did. What you then need to practice is delivery, not content.
The problem that people have is that they often associate reading with how you do reading out loud in school - you just kind of drone out the text in a monotone voice that's drab and boring. In reality, you can read and still make it dynamic and interesting and fluid and fun. Do practice that. Imagine your favorite speechgiver - a famous world leader, a CEO of a company, and so forth. A lot of them are reading their speeches. Think of your favorite newscaster or comedy anchorman. They are all reading off of the prompter and yet it feels like they're just talking to you.
It is pretty important that, when developing a dynamic reading habit, to get a sense of how you sound. Record yourself and play it back and see if it sounds like you're just talking to someone in conversation. That's how it should sound like. It shouldn't sound like you're reading a research paper. Figure out if it's the content that's doing this or if it's the delivery.
Once you practice this a lot - just reading dynamically - then you're going to start to know your talk so well that you won't need the notes, and you won't need the slides, and you won't need to worry about interruptions or anything.
Another thing: I don't write notes for every section for my talk: just the tricky parts that I stumble on. I often do it for the very beginning of the talk (Yes, I actually write "Thank you very much for the introduction. My name is Irwin and I'm happy to be here today" in my note slides because when I'm at my most nervous moment, I need to be able to start off without any ums, ifs, or buts), technical portions, and places where I have big "A-has" and "punchlines" and so forth.
Hope that helps.