1) If you know your stuff, you can wing it.
No professional speaker would ever go on stage unprepared. They may look as though they are ad-libbing, but a lot of work has gone into creating that impression. A speech needs to be carefully crafted, with a great opening and closing line and a strong message. That takes work. Every great speech has probably gone through dozens of re-writes and rehearsals. Never, ever, go on stage and try to make up a speech on the spot.
2) You need to know more than your audience.
This is one of the greatest concerns of inexperienced speakers, and they will avoid speaking to any audience where other experts may be present. It's rare that a speaker will know more about their topic than every member of an audience, and that's not what it's about. You're there to deliver your take on a topic, not to demonstrate your encyclopaedic knowledge. No-one else can do that but you.
3) If you have great slides, they'll carry you through.
If you use slides, great ones will help, but they won't outweigh poor delivery. You need to be able to engage and entertain regardless of the quality of your visual aids. Similarly, a great presentation style won't compensate for awful slides. Everything you do must be good.
Alan Stevens FPSA