I offered some advice upthread so please consider this my second-best GM advice.
Use ability checks to say "Yes, but..." to your players. When they want to do something
blatantly against the rules, if it makes sense as something their character might be able to do, have them make an ability check of some sort (details vary based on game system). On a success, they can do it; on a failure, they can't, and might even face negative consequences as a result of the attempt.
This works much better than just saying "No." By allowing a check, even a difficult one, you take much of the decision out of your hands. Ultimately the dice are deciding, but also the player can decide how much effort they want to put into it (expending special resources to improve their results; again, details vary by game system). It works well because even if the players fail, they still feel that they at least had a shot; and when they succeed, you know they "earned" it, and you're not just being a softie.
In most respects this is no different than a regular ability/skill check. Except that I advocate allowing check results to override even things that are phrased in the rules as absolutes. E.g.,
- You want to move farther than your speed allows? Yes, but... make a Dexterity check, and if you fail, you fall on your face.
- You want to concentrate on two spells at once? Yes, but... make a Constitution check, and if you fail, you lose both spells.
- You want to just totally ignore the mind-control for 1 round? Yes, but... make a Wisdom check, and if you fail, not only are you still mind-controlled, but saves to escape from mind-control are at disadvantage.
- You want to cast lightning bolt as a cone instead of a line? Yes, but... make an Intelligence check, and if you fail, then your spell hits you and no-one else.
The rules phrase those things as absolutes: a
lightning bolt is a line, end-of-story. I think everyone will be happier if you let go of such rigid interpretations and allow capable characters to break those rules a little bit.