I consider myself to be a successful DM. I could spend my time expounding upon why I think that, but the more important question is what actually makes my campaigns successful. If I had to choose one thing, it definitely wouldn't be my impeccable planning, my massive amount of effort, my unshakeable charisma, or my top-notch minis and props. (I have none of the above, haha.)
The single most important thing I do to make my games successful is choosing players.
I know many GMs advise prioritizing an open and welcoming table that anyone can join - and there's definitely something to that. A friendly and welcoming atmosphere IS important - which is why it's important who's at the table, who creates the atmosphere. A GM has a DUTY to be selective as to his or her players, as it is a vital factor in determining how fun the game actually is, which is, of course, the POINT of these games. To be fun.
I'm gonna try to give a brief rundown of what to look for (and what to look OUT for) in potential players, but the truth of the matter is, I choose players mostly by feel. It works really well (after years of DMing, I've NEVER had a player leave from strife or even lack of interest), but just makes it hard to quantify in a blog post - but here it goes anyway.
Things to Look For:
1) Rock-solid reliables. People who aren't afraid to make commitments, and who stick by them. It's key for any long-running campaign (weekly, monthly, whatever) that a consistent group of players be able to get together at fairly regular intervals. Long lulls and interludes dull interest and engagement, and missing party members tug at the fabric of the story. When you invite prospective players to your table, try to stick to people who can plan on being there regularly.
2) Imaginers. You want people who do more with their minds than browse social media, watch sports, and get wasted. Not that there's necessarily anything WRONG with the above - just that a person who doesn't bother engaging their imagination on a regular basis won't get much out of a tabletop RPG, for reasons that I hope are obvious. Look for people who read a lot, who discuss unusual topics, who have a variety of free-time activities.
3) The shy. I might be exceeding my self-mandate here - to give you guidelines for assembling the best table you can - but there might be something important here. I've seen tons of people normally thought of as "shy" or "friendless" really find their place at a gaming table. It makes social interaction easy, because it removes the sometimes-crippling focus on YOU as a person. You aren't YOU anymore - you're Bugaboo the Ranger, or Smervia the Bard, or whatever. People with social insecurity and anxiety often make really great players, because the game gives them an opportunity to be part of a group without regard for their appearance, race, occupation, sexuality, mannerisms, or fears. Some people really need that in their lives, and it might be something you can offer. (Still, take the following section into account, too.)
Things to Look OUT For:
1) Controllers. Players who constantly criticize the decisions of the GM (or worse, other players), just because they would've done differently. The core of tabletop RPGs is the making of meaningful choices in a shared fictional setting, and a player who acts as if the choices of others must conform to their own eats at the very core of the game.
2) Attention whores. Anyone who's always fishing for attention or causing drama in the social world has no place at a gaming table. If all they want is always to be at the center of the action, it will make it difficult for more self-controlled players to participate in the common game. Worse, attention whores will often CREATE conflict just for the spotlight-time it brings. You don't need that at your table, or in your life.
3) Lone wolves. By this I don't mean your garden-variety loner, who merely prefers to keep his or her own company much of the time. I mean the kind of person who doesn't give two farts about cooperation or common goals, but just wants to generally exercise their fictional freedom without regard for its place in a group context.
...) It would be a waste of your (and my) time to go through the more obvious warning signs in prospective players (verbally abusive, chronic anger issues, antagony with an existing group member), so I'll just say: don't invite anyone to your gaming table who you wouldn't invite to your dinner table.
Reader: tell me about the best person, and/or the worst person, you've ever had in your gaming group. What was (or should've been) the clue as to how they'd do at the table?