While 3rd Edition has cleared up the need for awarding extra experience points to characters for defeating monsters (for Fighters), for successfully casting a spell (for Clerics and Wizards) or for successfully backstabbing or sneaking up upon an enemy (for Thieves) by standardizing the Experience Point progressions for all classes and customizing Experience Point awards by the giving opponents a Challenge Rating instead of a base Experience Point value; there still gives rise for a desire, among some Dungeon Masters, to award players for their Role-Playing during a game session.
In the Dungeon Master's Guide, pg 168, under "Variant: Story Awards," it states, "...you need to set up a system in which you can award XP for accomplishing goals and for actions and encounters that don't involve combat." Below is just such a system.
As a base a Dungeon Master can assign Experience Points, for good Role-Playing, of 50 x Character Level at the end of each adventure or each game as the DM sees fit. This is by no means an astronomical amount but it gives us a base to work with. Where a first level character would gain 50 XP an 8th level character would gain 400 XP at the end of an adventure. By doing this, and using standard Experience Point awards, the PCs should reach the next level in 8 encounters, rather than 13.33 (as stated on pg 169 of the DMG). While, at lower levels, characters may Role-Play less and engage in combat more often, at higher levels PCs "...gain levels somewhat more slowly. Higher-level characters also tend to spend more and more time interacting with each other and with NPCs, which results in fewer experience points over time."
Characters that engage in social interaction, whether trying to persuade the king to loan the party his Scepter of Lordly Might or by bluffing the Lord of Thieves into believing that they are the true Slave Masters, should, at the very least, gain experience through a variant award system.
Another way to award role-play based Experience, though tedious, is to base the awards off of the success by which PCs succeed in using their social skills (bluff, diplomacy, sense motive, etc). The formula for such an awards system might proceed as follows; 10 x the difference of the skill check (roll - DC); the total of which is multiplied by the level of the character (failures are not counted and a character can never loose Experience Points by failing a skill check. If one uses equal level "opponents," on average, a first level character will only gain 5 EXP, every other skill check, through this method. A 1st Level character with 4 ranks in Bluff against a 1st Level Aristocrat with 4 ranks in Sense Motive will, on average roll 14.5 vs. the Aristocrats' 14.5 Sense Motive. But as the characters rise in levels this will give rise to greater margins of Experience Point gain.
Using Skill Based Awards is tedious as the DM must keep track of successful skill checks and by how much they succeeded while running the rest of the game.