Give the players Belief Points.
Every DM worth his salt knows that showing > telling. But what's better than showing the players something? Let them DO something. A player character that's roleplayed at least half decently will belief in something. The right to freedom, the absoluteness of the law, that lying is wrong, giving money to the poor, that sort of thing. Having your players live up to this can make for a lot of fun roleplaying, but it not always comes naturally. But if they know that belief can reshape the planes... well, you'd be surprised at how eager they become to stick to their beliefs. Hence, Belief Points.
Taken from the Planeswalker's Handbook (Planescape book, page 142-147), Belief Points are given to the players if they stick to their beliefs, but at a cost. They are handed out at the DM's discretion and should not be granted frequently. For example, someone might believe that you must always answer questions truthfully, but that does not mean that if they answer a question truthfully that they would gain nothing from if lied about, you do not get a Belief Point. But if this were to come at great cost, personal or otherwise, then you do get them.
For example, if a Fiend were to ask "Am I a fiend?" and the character answers this correctly, this does not grant Belief Points. But if they were to be asked something that would result in the character's friends or allies being hurt based on the answer this would grant belief points. You can award one point for adhering to a belief but at not too great a cost, two for cases when there is a significant cost, and three in massive costs, perhaps even at the risk of the character's life.
But what do Belief Points do? By spending one a character can automatically succeed on any die roll (attack roll, saving throw, skill check or whatever you allow). This indicates that things go better for those with conviction. If a player spends three points at the same time however they can ask the DM a simple question regarding where the player is, the hazards they might face, what the enemy might do next and so on. This symbolizes a gut feeling, or a leap of intuition. Of course, the DM is not required to answer or can alter the cost to reflect the importance of the information.
Do make sure that the players can handle this. If you have conflicting beliefs in a party players will end up at odds with one another when trying to stick to their beliefs, and that's no fun for anyone involved. Furthermore, acting on a belief should not undermine the party or the progression of the plot. Also, players should not be forced to stick to their beliefs, but not doing so just does not grant Belief Points.