I sometimes ask for rolls for various skill checks solely for the purpose of creating tension.
You, as a GM, ask for a perception roll and then just go "hmm... okay," and move on, and they will wonder what they have missed. Do not overuse this trick though, as it can backfire on you.
However, if a spot or other sort of perception roll is called for, and the player does not make it, you can always just give them partial information.
For example, if the DC for overhearing a couple of guards talking is 20, and the player gets a 15, then just give him snatches of the conversation (maybe one word out of 5 that a successful roll would have given him).
That way, you up the importance of good rolls, and yet do not give everything away on a failed roll either....
You, as a GM, ask for a perception roll and then just go "hmm... okay," and move on, and they will wonder what they have missed. Do not overuse this trick though, as it can backfire on you.
However, if a spot or other sort of perception roll is called for, and the player does not make it, you can always just give them partial information.
For example, if the DC for overhearing a couple of guards talking is 20, and the player gets a 15, then just give him snatches of the conversation (maybe one word out of 5 that a successful roll would have given him).
That way, you up the importance of good rolls, and yet do not give everything away on a failed roll either....