I chose "all the time," but it's not really accurate ... it probably happens every two or three game sessions. I adopted a concept called "drama dice," where a particularly cool idea, slick move, or brilliant plan could get a bonus, e.g....
A neat stunt or idea (dive through the plate glass window and tackle the cult leader from behind): +1d4 applied either to the to-hit roll or damage roll
A really neat stunt or idea (tie a rope around an incapacitated henchman, using him as a live counterweight as you bungie-jump out the 10th story window): +1d6 applied to your skill check
An incredibly neat stunt or idea (have an army of ghosts come pouring out from behind you as you surprise-charge the orcs standing patrol on the shores of Osgiliath): +1d8 applied to the party attack rolls for the round as the orcs stand around frozen in shock and horror
You also get an experience bonus of (your level x 10 x the drama dice result).
The caveat is that you can only get one bonus of each type per game session, and successive ones have to be larger -- i.e., if you do something really cool at the beginning of the session that gives a +1d6 drama die, you can't go back and earn a +1d4, but you could still potentially earn a +1d8.
For more mundane "favorable/unfavorable circumstances," I just use the generic +2/-2 ... which is more reliable than +1d4, but doesn't give you an experience bonus, either.
-The Gneech
