It's tougher to be a solid public speaker than it looks. I've given talks to big rooms, gyms and moderately sized conference rooms. I rarely get nervous but occasionally it hits us all. I do have some notes on watching great speakers in person and learning from my own mistakes. 1. Do NOT ask for a 'show of hands' from the audience. Even some practiced speakers do this. They think that it engages the audience, but it…
#2: Absolutely. It's better if you can do without, but it's also a matter of how much time you have to prepare, and how dense the material is. If I'm giving a hard-core technical talk for 45 minutes, I'll sure as heck have a few notes handy. I prefer to go without, but that requires a lot of rehearsals. (45 minutes without notes is probably ~8-10 days of solid work. With notes, 4-5 days)
#3 That just means you've got to get better at reading your audience :) Seriously, if you feel a disconnect, shake things up. Do something unexpected. Have a joke ready. You do not want to lose your audience, ever. It's very hard to get them back.