One of my day jobs is being a personal trainer. It turns out that one of my current clients is a gamer.* He mentioned he'd be running a D&D game the afternoon after a session.
We chatted about it a bit, and I ended up giving him some advice.
1) Don't Let On When You're Making Stuff Up As You Go
Often I've played with GMs who'll pull back the curtain after the game and tell you "This was in the adventure, this was not. I made this up on the spot, I made this up later." I've had GMs do this mid-game.
I'm guilty of it occasionally myself.
But generally, avoid it.
It's so much easier as the PCs to keep your suspension of disbelief if this doesn't happen. It's easier to maintain confidence in and awe over the GM's ability when it doesn't happen.
It's fine to toss them a cookie now and then, and revel in how well you set things up. But default to keeping it secret. Find another GM, another player who isn't in your game and won't be in your game, and tell those people. Respect your players enough to let them experience mystery in what was planned, what was unplanned, and what they created.
2) The PCs Start Knowing Each Other
He was going to run a first session. I suggested something I learned over the years works well - have the PCs all know each other from the start. None of this "meet in a bar" scene. None of this "you see each other on the street" scene. That always, quite logically, ends up in intra-party conflict and fighting and arguments backed by "but I was just roleplaying my guy! He's a Bad Tempered Loner who is very sensitive about people looking at him or talking to him in bars or on the street!"
The heck with that.
Instead, all of the PCs know each other. Or at least know multiple other PCs in the group. Have two elves? Fine, they're cousins. Brothers. Went to bow school together. Whatever. Even if one is a NE wizard and the other is a CG ranger, they've got a long-term background bond. Stuff like that works - you just announce, "You all know each other, and you're all friends. Figure out how now or in play."
This will save at least half of the first session, too. I've gone as far as "You all know each other, and you've all agreed to go on this quest that one of the PCs was asked to do." That way they don't even need to waste time trying a) figure out what the adventure is and b) decide if they want to go on it. They don't want to do a), and they absolutely want to do b). So just start there.
I also told him to game with friends, and our rules on inviting new people.
And I also told him to keep his chest up on front squats. I'm not just a fellow GM, but I'm also his trainer, after all.
* One of my former clients was a gamer, too, which is partly why he became one of my clients. That's another story.