Well, I do believe that I am fairly experienced in what not to do, so I may as well share what I've learned from that.
GM Tips:
First thing: For the love of god, think your idea through first. Come up with an intended plotline, but be okay if the players decide to smash it into little bits. Make a map - nobody I know likes trying to explore the outer edges of the world likes to run into some random bullshit that was completely made up on the spot. Poorly-thought-out games tend to die quickly, no matter how fun they are.
Second thing: never, ever, ever, ever, ever make too many major changes to your game within a short time. I did this with the art style/update speed in Prince of Toast the first time around, and it pretty much killed it. Which leads me to...
Third thing: Do not obligate yourself to do something you know you won't do well. I tried to update that Prince of Toast game once a day or so, but a bad case of school and lazybastarditis dropped the quality to the point where people were asking what the hell things were instead of making suggestions.
Fourth thing: discuss the idea with experienced players and such first. Even if you've been doing RP since you were 3, it never hurts to get an alternate view on things. Plus, it lets people know that you're going to start a game up soon and that you've actually put some thought into it.
Fifth thing: If you're a first-time GM, do something simple. Use a system that's already been built for you instead of trying to build your own. Learning the general ropes of GMing is hard enough without having to try to balance your own system.
Player Tips:
First thing: Put more than two seconds of thought into a character, suggestions, etc. It contributes a lot to the quality of the game when the characters aren't all "Princess Bob who, uh, likes flowers and shit."
Second thing: Mary-Sues and memes. A small dose of either in a character isn't too bad, but it's like salt. If you add too much, everything is going to taste horrible, and nobody will be able to take the game seriously.
Third thing: GMs are people too. They will make mistakes, the will miss things, and it is your job as a player to not hate them for it. Poke them a bit, show them what you think was wrong, and leave it at that. Do NOT try to convince the GM that they were wrong, because it usually ends with rocks falling. Or thirty-page-long arguments about the logistics of quantum acceleration in a pseudoscience game.
and one last general tip:
Spellcheck and the enter key are your friends. I've seen both GMs and players try to play without either, and it gets ugly.
So this doesn't answer the original question of the thread at all, but it seems like the right place to put it.