I award karma at the end of each session, because we often run long missions, and I don't want to forget cool stuff that happened along the way. I award 1 point for survival in sessions where it's an issue, 1 point (or more) for each mission objective, 1 point for making new contacts, and 1-2 points for cool stuff. Altogether, it usually works out to be about 4 points per session, more at the end of a long mission.
I handle "cool stuff" a bit differently than the rulebook recommendations. I don't give individual awards for role-playing, humor, and so on. Instead, I ask each player to describe something cool that happened during the session. I find that players are better at giving recognition to each other than I am. After some sessions, there's so much cool stuff that we do another round of coolness nominations (or I add my own) for a second karma point.