One of the things I liked most about the 3:16 game (link) we played awhile ago was the game mechanic for adlibbing character background and gaining bonuses in doing so. For those of you who didn’t play in that game the system let you automatically succeed in a skill check or even an entire encounter by sighting some piece of character background (previously unmentioned and pretty much made up on the spot) that explained how your character gains this advantage, with the converse being that your could also cite (or have cited for you) a negative trait that would allow you to survive (but not win) an encounter that was going particularly badly.
I would like to include a similar idea into our current game, namely providing suitable roll adjustments for sighting new pieces of character background, for example you may receive a +2 bonus to diplomacy roles with a given group should you provide a suitable anecdote showing your extensive experience with the group.
To balance this out any bonus received on a given skill could later result in me applying a modifier to the DC for a related skill check later on, for example the next group you talk to may not get along so well with the group you got the last bonus on, so I might raise the diplomacy DC by +2 as a result, with me supplying appropriate background for this group not receiving your presence quite so well.
Only one positive and one negative effect could be applied to each player in any given session, although the negative effects can be saved for use at a later date I wouldn’t want to punish anyone by stacking up multiple negative adjustments on the same day.