When I flew tours in Alaska, it wasn't uncommon for a passenger to slip you a twenty after the flight. Usually if you make it known that someone gave you a tip, others feel need to follow suit.
I've also had passengers ask if its appropriate to tip the pilot...anyone have a good witty response to that? I never had a good answer other than "uh, sure". The more at ease and comfortable you make people feel, the better the odds of scoring some change afterwards.
Now I fly corporate/135, and I feel lucky if I get a smile and a "thank you"!
B