Bluff states
You know how to tell a lie.
Check
Bluff is an opposed skill check against your opponent’s Sense Motive skill.Deceive or Lie
If you use Bluff to fool someone, with a successful check you convince your opponent that what you are saying is true. Bluff checks are modified depending upon the believability of the lie. The following modifiers are applied to the roll of the creature attempting to tell the lie. Note that some lies are so improbable that it is impossible to convince anyone that they are true (subject to GM discretion).
Retry? If you fail to deceive someone, any further checks made to deceive them are made at a –10 penalty and may be impossible (GM discretion).
Table: Bluff Modifiers
\$
\begin{array}{|l|r}
\text{Circumstances}&\text{Bluff Modifier}\\\hline
\text{The target wants to believe you}&+5\\
\text{The lie is believable}&+0\\
\text{The lie is unlikely}&–5\\
\text{The lie is far-fetched}&–10\\
\text{The lie is impossible}&–20\\
\text{The target is drunk or impaired}&+5\\
\text{You possess convincing proof}&\text{up to }+10
\end{array}
\$
Here are some of the issues I've dealt with…
Due to it being used to "abuse" his character (at a different table, not mine), I had a player that hated bluff so much that he stated, "My character believes that everyone is against him and doesn't believe anything anybody says." When I mentioned the logical problems with such a statement, he replied, "you know what I mean". Then I told him that this would invalidate part of the bluff skill in the game. To which he said, "Bluff isn't mind control I don't have to believe it." I ended with, "if you fail your sense motive check, the bluff skill states your character believes what was said." He ended with, "but it doesn't control how my character will act on what he believes is true."
I don't want to cheapen the Bluff skill, but at the same time I'm not looking to control his character's actions.
The other situation happened when a devil bluffed a Paladin. They party was adventuring in a desert terrain and met a devil that claimed he had been turned into such by powerful magic and asked if he could accompany the party to get cured. The devil had a massive bluff skill total, and so he fooled the party. They asked the devil a lot of questions and used some spells to see if he was lying, the devil passed them all. Then the player of the Paladin out of game stated, "I don't believe this devil, but my character does. What precautions can my character take while still not being false to my character believing him?"
He also stated, "If my Paladin believes him I don't think he would ask the character to tie himself up."
In D&D…the impossible, is possible. How can a GM use the bluff skill correctly while also taking into account how a player character is to behave towards what he believes to be true?
No! A Chaotic Neutral Barbarian is not the answer! 😉
Let me add this last part. Sometimes these bluffs are happening in the middle of life and death situations, i.e. the PC's have just entered a room where a succubus appears to be a woman who is a wounded captive, while another NPC adventurer who believes she's evil (because he didn't fail his sense motive) is attacking her and won't stop, she successfully bluffs the PC's for help. I hope the players wouldn't metagame a encounter like this!
Best Answer
Offical rules and clarifications for the Bluff skill
There are many good answers with general advice already, instead, I will show you what the game rules advise on how you should be handling this since the core rulebook is vague about social conflicts in general.
This is what the developers have to say about how to use Bluff (from Ultimate Intrigue, p.182), which I will quote it nearly fully because almost everything is relevant to this discussion, as such, I apologize for the wall of text.
The book, straight away, admits that the core rulebook has short descriptions and that the following are clarifications and not new optional rules. Either way, the GM is free to ignore this text, obviously.
Then, it describe many possible scenarios of characters using Bluff that could (and will) come up during play, be that on social conflicts or not:
Summary
There are many situations described on the text, but a few points to take note:
House rules
Personally, I never roll for Bluff when GMing NPC's (unless its being used in combat), I use 10 + Bluff bonus as the DC of the PC's Sense Motive check. Those checks may be asked on demand by the players, and are rolled in secret (even if the NPC is not actually bluffing). Also be affirmative on the result: "She is speaking the truth" and not "She appears to be speaking the truth". Unless your intent is exactly to get them on the edge and suspicious of everybody they meet (which is fine for a horror game).
When discovering this bluff may be important to the history and has roleplay potential, I will do the opposite and roll all players Sense Motives against the bluffer (also in secret), using 10 + Sense Motive bonus as the DC for that check.
If one of the PCs pass on that check, I will inform that the PC(s) notice that something is not right in what they are being told, but not exactly what, they will have to roleplay their way out of it and possibly make more Sense Motive checks, along with some Bluff and Diplomacy checks. The NPC may simply be hiding something, they may be afraid of being discovered, or are afraid of their boss, or are sad or angry about something they just mentioned, and so on. They have something to hide and that something could be discovered by the players, so a Bluff/Sense Motive check is warranted.
This is similar to a Skill Challenge to those familiar with it (D&D 4e), or the Social Conflicts rules. And also similar to how these rules are been implemented on the second edition of the game (currently in beta) and work fine for me.