Rules for the GM
Rule #4: The players should be right some of the time. They'll enjoy the game more if they aren't always wrong. If the players come up with a good idea, let it work. Even if you originally thought that there would be only one way they could solve the challenge you gave them, there's no reason you have to make that one way be the only way. Reward the players for creative thinking. After all, roleplaying is supposed to be more of a creative exercise than a problem-solving exercise, right? If the character has to keep trying idea after idea until they hit upon your solution, then the game won't be much fun and the story won't go anywhere. If the player only needs to come up with one good solution, then the story should move along at a good pace.

Along the same line of thinking- If a player decides to prepare for some threat and goes to all the trouble of explaining the preparations, then by all means have those preparations come in handy or at least have some involvement in the story. The things that the character has prepared may not come in to play the way the player thought it would, but the player will still enjoy the game more by seeing their actions have some effect on the story. Once again, this rewards creative thinking. For example, your player decides that his character's home needs a new upgraded security system in case the game's main villian decides to sneak in and attack him in his sleep. The player goes to all the trouble of describing this elaborate new system. If you have no plans on having the main villian doing something so mundane, there's no reason that another NPC couldn't decide to sneak in and steal something from the character. With his new "stealth suit" the NPC was sure that no security system in the world would have been able to stop him...