Never address anyone by "Mr. so-and-so" except in a friendly joking well. First names mostly, last names in a bold way sometimes, and never by their title.
Everyone is a peer of yours.
No corporate speak, ever.
Never let yourself be intimidated. Intimidated people aren't fun to be around.
Keep it light.
Stay on topic -- it's really not fun having having people drift into nonsense and not getting important deliverables down. [*]
[*] I struggle with this sometimes. We all struggle with something. My mind goes on tangents, sometimes useful, sometimes not. I've been working on it a lot lately.
Make all your collateral focused on the buyer. Identify risks, yes. But make sure you leave them with a broad strong promise. Don't talk about yourself too much. Nobody cares.
Don't be too formal.
Do be professional.
Professional isn't accoutrement or nonsense. Professional is being focused on understanding the buyer and delivering the results they want.
Winning is fun.
NEVER bow and scrape. If you're a clear subordinate, people can't let their hair down. They have to keep up "authority mode." If you're true peers, you can relate as true peers and get an honest dialog going. People don't get honest dialogs going with subordinates. So, never bow and scrape. That's the biggest mistake new people make. Everyone's a peer. Keep it fun.