One of the hardest things a GM has to learn to do is throw away their ideas. Players will abandon plots and almost always come up with solutions you didn't think of. Some GMs fight this by simply shutting down players or forcing them onto THEIR path. When this happens the game stops being cooperative and either turns into one of a few things, an analog video game where you are limited to what the "designer" had in mind, a game of animosity where the players feel like you are "against them, or just a book you REALLY want to tell your friends and none of that is what a tabletop RPG is.
Ok yes some games can be antagonistic where it is the GM vs the Player, but honestly we've all grown in gaming since the 70's and have found much more better ways to play a game than just having a modified war game. That plus the fact that if you're already reading this odds are you aren't the stubborn single minded type to see a game as just you vs them and if you are oh well. Anyways back on topic, letting go. Lots of gamers talking about the "rule of yes" and yea it covers some of this, but I'm going to go deeper into the "rule of fun" and the "rule of common sense" both of which are completely necessary if you want a game to be memorable.
The Rule of Yes
This should be short, but in case you live under a rock without internet yet somehow are reading this the rule of yes basically tells GMs to say "yes" to their players and their ideas as often as possible. Its a good place to start both for training new GMs and rehabilitating bad ones. It turns off the "I'm always right and I know everything" mentality some people have while running a game and helps foster creative play. The problem being by itself it also can create GMs that are doormats. Sure most that write about this add in the "as long as it's not unreasonable", but for a new GM many things can SEEM reasonable at the time only to blow up in their faces. It also allows for players to proclaim, "but you said yes last time that means its a rule now". Experience teaches us how to handle this, but as a tool for new GMs that experience isn't there.
The Rule of Fun
This is kind of like the rule of yes, but takes it a little further and asks if saying yes will make the game more fun. The trick here is that the question isn't will it make the game more fun for the ONE person asking, but will it make things more fun for everyone including you. This is where the rule of yes gets abused so many times. You have one player who in truth doesn't care about anyone else having fun at the table as long as THEY get to play and be cool. Saying yes to that guy having a friend in the city guard that can get the party out of trouble might be fun for everyone, but this comes into knowing your player because that same rule can be used to get THEM out of trouble and screw the rest of the party.
In a very recent story from one of my games one of my players wanted to have a Wyvern (Basically a small ride-able wild dragon) as a pet. I said they COULD if they venture out into the wilds steal an egg and raise the beast from birth. This gives the party an adventure, one where the single player couldn't do it alone which also means saying no isn't on my hands but the group as a whole. At the same time another player was coming back into the game with a new character and I told them that they needed to write in a backstory that would let the party naturally come across them rather than bending fate to "stumble" onto this character in the wilds. He hears they are questing for a Wyvern egg and decided he's from a tribe that raises the beasts as mounts and even has one as in his culture they are given an egg at birth and are raised together (see how he heard all the "Rules" for getting a Wyvern and just gave them to himself). This is a player that is a massive red flag. One he would turn an adventure into just finding him like he is the reward and thus "Special" while also giving himself something for free that someone else knew they had to work for. (There's also the fact that he gave himself the "reason" for being with the party would be they would be capable enough for him to hire them for his quest.)
The Rule of Common Sense
Notice above that the while both players were wanting the same thing that it is obvious that the second player is trying to "get something for nothing" that's common sense. Look there's no way to teach experience in running a game that involves handling people. The only thing you can do is trust your gut. This sounds obvious, but when you're dealing with rules of yes and fun it's important to mention. You see lots of nervous first time GMs will think that since the world "rule" is in it they have to do it. IF you gut tells you no then say no. This actually applies past these rules as well. As massive as some game systems can be, no game has a rule for every possible outcome and sometimes you jsut have to wing it. Hell in my opinion as long as you know the core foundation of a system you can eventually wing most of the arbitrary rules as well. That's not to ignore the rules, but don't stop a game to look through a book for fifteen minutes when your gut tells you different. Sometimes (ok lots of times) you're wrong, but if people have fun then it doesn't matter.
Bonus: It's OK To Be Wrong
When you're dealing with your gut sometimes you chose "wrong". By that I mean sometimes you make a call that the rules don't allow or disallow something the rules say technically should be ok. Other times this means you say yes to something that your players abuse or you award an item that heavily throws the game off balance. The main thing to do here is to look at the game and ask one thing "is the group having fun?" as long as people are having fun just go with it (at least for a while till they get bored of it). Just like the rule of fun notice how I asked if the GROUP is having fun. If you break the game and one players is having a blast while everyone else hangs back checking their phones then something needs to be fixed. When it comes to a rules flub it's pretty simple, just own up to being wrong tell your players that you messed up and that while it won't undo anything they already did you will try better to keep things right from now own. Items and rewards are a bit trickier, but you can either have a talk with that one player about how you messed up and find a way to meet in the middle where they get something, but it's less broken or find a way to eventually either have the reward turn into as much of a pain as it's an asset or in dire cases ways to make your player just lose it entirely.
I don't suggest arbitrarily stripping rewards from your players, but some again think their fun is more important and refuse to let go. In that case find a way to have it happen "naturally" within the game. Let them screw up in a way that loses the reward or they fail to save it. If they succeed wait a bit and try again. Do NOT try again right away then it will just come off as picking on a player and feed into that idea of antagonistic game play. Which as I've said before if a player is refusing to look at everyone's fun then they probably are already thinking this way and will just reinforce their bad habits.
Not For Everyone
I will say up front that this style of play isn't for everyone and I've had players that insist on stopping the entire game so they can site a rule that disproves what you allowed because they have to be proven right. The thing is all that's doing is satisfying that one player while everyone else does nothing. Funny enough this happens more on instances that won't even make a lasting impact on the game, but that doesn't matter. If you're fun is proving you're right and others are wrong you're looking for an antagonistic game of winners and losers. Some people want that type of game and I'm not one to stop them from having their fun too, just not at my table.