Back in 2004, Vincent Baker's Lumpley Games released Dogs in the Vineyard, an indie role-playing game set in a version of the Wild West loosely based on the Mormon State of Deseret in pre-statehood Utah. Players play "God's Watchdogs", individuals working for the Mormon Church who travel from town to town, performing a myriad of different tasks.
One aspect that many people remember about Dogs in the Vineyard that it's rules promoted the idea that Game Masters should always "say yes, or roll dice." This piece of advice is pretty simple. A Game Master should either go along with a player's suggestions or at least give them the opportunity for the suggestion to occur based upon a roll of the dice.
Some individuals within the gaming community have taken this idea a step further, believing a Game Master should always say "Yes" to their players. Others have taken the basic concept and expanded upon it, creating two distinct variations: The "Yes, and..." and the "Yes, but..." rules. It seems like the very idea of telling a player "No" has become something only bad people do and they should feel bad for doing it.
Thankfully, I'm not the first person to say how ridiculous that notion is. While I believe a Game Master should say yes to his or her players, since it allows said players to have more influence upon the game and takes some of the creative weight of the Game Master's shoulders, they need to occasionally say no as well.
Now that we've introduced the idea that saying "No" to your players is just as important as saying "Yes" to them, we now have another question to answer: When should a Game Master say no and how should that Game Master say it?
Generally, I believe a Game Master should say no to a player when said player wants to do something that will create problems for the game and the other people playing at the table. Let's imagine you're running a game with a group of people and one of them is playing a character who happens to lean more towards the evil side of the alignment scale.
They want to do something that you know will not only upset the other players, but will make it hard for them to justify why their characters are still adventuring with this person. Because of that, you should probably tell them "no" and possibly explain why. However, you should keep it short and to the point to keep the game moving.
A Game Master should also say "No" when a player wants to do something that clashes with the game world being used. An example would be a player wanted to create an elf in a modern setting with no magic or magical creatures.
Don't be harsh when you tell a player "No". Remain calm, cool, and collected. Explain yourself if need be, but keep it short and to the point so you can keep the flow of the session moving at the correct pace. As the Game Master, you know what's healthy and not healthy for your group and the game you're trying to run. While saying "Yes" to your players is good and you should do it as much as possible, don't be afraid to say "No" every once in awhile.