Being a Dungeon Master can be hard. There are so many rules to remember and use throughout our sessions. On top of that we come up with our own creatures that often follow their own rules. Keeping all of this in your brain at once can be difficult, especially for the new Dungeon Master. For this reason I often find Dungeon Masters doing whatever they can do in order to avoid having to remember. Part of this is what I call “Intuitive Dungeon Mastering”. Well, I actually shamelessly reused the term from someone I know, but let's not sweat the details. The idea here is that things should just make sense to the Dungeon Master. It's a simple idea but like so many simple ideas it can be easier said than done. And so, here is my advice when implementing the concept. As usual, it's a bit biased towards D&D 5th edition, but still applies in other rule systems.
Keep Rules Down
If you are trying to remember a whole role system, superfluous rules are the enemy. Commonly used rules are best remembered and if you can reuse them, it makes things doubly easy. They probably make sense to you already, and it's less to remember. Of course, while we often can reuse rules for our own evil ends (stealing spells, applying clever use of contests, etc.), it isn't always possible. The more alien the ability or situation the less likely it is that there is a rule that will apply. It's worth noting here though that the presence of spells makes recycling rules far more potent than otherwise possible. If we are designing a new rule, I think it should be because the situation is very alien or we want to add complexity. Complexity isn't always bad, such as when you are trying to model giant armies clashing or the new powers of your godly players. However, not everything should add complexity. Some things are natural extensions of what already exists, or we can use what already exists easily to do what we want.
Criteria for New Rules
Okay, well, we failed step one. We can't re-purpose an existing rule and apply it into a new situation. That, or it doesn't actually work in this case. Don't panic. Everything is fine. We can make a new one! Yeah. Only problem is that making rules is a bit of an art. There are awards for game design, and I don't have one of them. We want the rule to be easy to remember, to make sense in context, and to not forget it during play. Not forgetting a rule tends to be easier if you made up the rule since you have an intuitive understanding of the thought process behind it. However, given a bit of time it can be forgotten if it doesn't make sense. For those reasons I find it helpful to keep those considerations in mind when making new rules.
Make One Up On The Spot
Rules made up in the heat of the moment can be surprisingly versatile. Since they are done in the moment, they often make sense for the context that they were born from. You use your understanding of the rules to make something that probably makes sense to you in the moment. That is, of course, if you didn't have a complete mind blank. That's why you can't rely on coming up with a rule in the moment. If you get caught and need one, roll with something. My experience is that anything that keeps things moving is better than consulting rules for half an hour. If you realize there's a problem, amend it going forward based on your reflection after the session. Still, what I often want to do is capture that simplicity and intuitiveness. A rule thought up on the spot may not be final. It was fine to keep things going but you can further refine or simplify it afterwards after reflection on it. Rules in the moment can fit perfectly into a situation and be simple. However, they can also be half-baked.
Just For You
If you are running a session, chances are high that many of the rules you'll be using are meant for you and you alone. And you know what? That's fine. It makes things easier in a way. All you need is for the rule to make sense to you. Don't worry about making it fit into the rule system or how someone would understand them when you are in the heat of playing a game. If it makes sense to you, and you can easily remember it, what else do you need?
Rules are an abstraction. Your players typically won't know what determines your decisions in many cases. That's part of the fun too: not knowing how things will go. They are a tool to get you to take input from your players, add some input from yourself and return it back to your players. That, or give your players something that they will then begin to shape. What mechanism decides these things often isn't important. Whatever helps the Dungeon Master do their job is fine.There is a caveat though. To make a decision players need some level of understanding of what is going on. They may not know the rule, but they will need to know roughly how crazy a certain action is. If anything can happen at any time for any reason, how can we expect them to know what options are available?
So Give Up Making General Rules?
No, I didn't say that. I find that there are some situations that more often lead to intuitive rulings or rules. Sometimes there's nothing to be intuitive about because it never happened before. What is a problem is coming up with a rule that doesn't work for you because it is too complex, or too easy to mix up with another existing rule. It's a careful a balance between not reinventing the wheel every time, and knowing when a new rule is necessary.
If you are playing D&D, thinking about DCs is a good starting point I think. It might not even use a stat from the players, such as a death saving throw. However, they give you a quick way to determine difficulty for a task and a mechanism for resolution. They are the central resolution mechanic and you probably don't want to try to subvert it when it makes sense to apply it. The results of doing so can to be complicated. Piggy backing off of this rule feature also typically means that its easier to remember the rule. It's also amazing how much the existing spells can give you. Often they might need a bit of retooling but again, they make for an amazing starting point. Knowing how hard it is to do something is often one of the big questions in many situations.
Part of what I'm saying here also related to creating rules through rulings. If you decide that a certain rule applies in a situation where it may not be obvious, your ruling is establishing an intuitive rule. You can decide that if a flying creature was killed in the air, fell down 20 feet, and hit someone on the ground, they both will take the equivalent falling damage. That would be a great example of an intuitive rule. However, since it was moving before it got hit, you might put an extra 1D6 of damage. It had extra speed, so why shouldn't there be more damage? Such an argument is intuitive, and makes sense in context. Neither is wrong, but it's the kind of thing I'm talking about here. Depending on who your players are, they might always expect that damage now, or they may just be happy it happened once and not rely on it later. Many times these kinds of rulings lead to rules.