Twelve things I wish I had known before running games

A few other bloggers started this (here and here) I changed it up because I have been running games now for about 33 years now and I can barely remember when I was 12.

In no particular order - and I am not sure these are even good, just the first that came to mind - here is my list. I would say this is what I strive towards, as I bet I rarely follow all these ideas but I would like to.

1. Read all the game systems you can find - you can learn a great many things from reading how others accomplish things.
2. Do not read other games, this will only lead to more tinkering and you will forever be writing and rewriting house rules or, God forbid, your own gaming heartbreaker. You will spend more time tinkering than playing.
3. Listen to the players at the table, let them talk among themselves and never, ever forbid it. They will give you tons of ideas to riff off of that never occurred to you and this will only make your game better. Those four or five folks sitting at your table are just as creative as you, plus, hell, four brains are better than one, they will have great and whimsical ideas that will only add to your game.
4. The game is about problem solving, not combat. Always presents problems for the players to overcome through creative problem solving (not through dice). Especially for younger players this will create a strong pull towards the game. Requiring the player to use their brain instead of dice mechanics to resolve situations and problems is inherently engaging.
5. Do not be afraid to fudge things behind the screen in the name of making the game better, sometimes killing the party's fighter in an amazing bit of sacrifice-to-save-the-world can be the most amazing and memorable thing ever. Also, that nutty, crazy idea the player had that somehow completely destroys Strahd in one round despite what the die mechanics say can really make an outstanding and memorable scene that players will talk about for years.
6. Never map out things in details, think in terms of generalities with only a general outline of what you want to happen. Spending too much time planning and then having it derailed (see #6) will exhaust you and make running a game a chore and no longer fun.
7. Nothing survives contact with the players. Nothing. Be prepared for the unexpected and never be deadset on your idea coming to fruition. 
8. While OSR character death can be interesting, give players a chance to breath life into a character.  Long-played characters can be amazingly fulfilling and make a player want to come back for more.
9. Not everything needs to be explained, let some of the mysteries lie. These are powerful tools to use later, come back to them and the connection will breath life into your game.
10. Treat monsters as NPCs and not cannon fodder. If an interesting thing happens during an encounter, toss in a quick detail - maybe this particular goblin gets a scar across his cheek. Have him come back six sessions later with a large friend, looking for vengeance, when the players least expect it. #MonsterLivesMatter
11. Treat NPCs as characters, give them a few sentences of background, dreams, wishes, etc. Give them a unique quirk (a twitch, accent, a scar, a favorite drink or food, maybe a particularly unique tobacco they smoke) that will help define the character for the players. Obviously, do not do this for all of them, but the ones you want to showcase, perhaps one or two a session.
12. Random Tables can be great but use in moderation, they can add fresh life into a vanilla world but they can also induce chaos. Also, making your own random tables - particular to your gaming world - can really be fun and less chaotic.

Bonus thoughts:

• Describe scenes in generalities and let the players dig for details, this will produce a more interactive game world where the players feel engaging and involved.
• Read published adventures but never run them as is, read them and make them your own. This will make the world yours and unique to you.