OTOH, I've done a number of talks at various user group meetings, and conferences like All Things Open, and I prepare for those a little different. I try to be very prepared, and I do use slides (and/or code) but I still don't try to prepare a "word for word" speech or slide deck. I make slides that have bullets that I treat as "remind me what to talk about next" and maybe include expansions of acronyms and some links that people will find useful afterwards. But even then I try to make it so that the majority of the words coming out of my mouth are generated on the spot.
I was taught a long time ago "don't read the slides to the audience" and while I don't always adhere to that 100%, I really do try to just use the slides to control sequencing and to jog my memory about key points. I just need to know the material so well that I can talk about it in depth, with only the high points put up on the slides. I don't always achieve that goal, but I always feel comfortable during my talks, especially by about 45 seconds in, once I'm in the flow of things and any "nerves" have settled down.
The last thing I might add is this: if you do "go blank" or have any kind of gaffe, it's OK. Everybody has those sometimes. My advice is to just own the moment and don't get to wrapped up in it. If you lose your train of thought, just say "Oh. Well, guys, I just totally lost my train of thought there. Give me a sec". If you're comfortable cracking jokes in front of a crowd, feel free to make a joke of some sort. But if you're not, don't worry about that. Otherwise that itself might come off as very artificial and forced. Anyway, if the thought doesn't come back to you after a brief pause, just say "OK, forget that, let's just move on to ..." and launch into something else.
You might also find practicing improv comedy to be valuable. I did one improv class and think it was a very good thing. I just wish I had time to go back and do more of it.
I like to prepare. My best speeches were the ones where I wrote small ideas I could potentially say on post-it notes, and stuck them to a large wall.
Then, I re-arranged, edited, added, deleted, etc. until I liked the story line.
Then, (a la Tim Ferris), I break it up. I practice each 10 minute section separately, being able to deliver each 10 minute section 10 times in a row.
And just remember: presentations are not 'What do I say next?' but 'What do they need to learn next?'