Hand out your cards to all passengers
I just found this forum and wanted to share something I do when I fly. I ordered business cards from an online printing company- I think I got about 200-300 for about 25.00, can't really remember, but was well worth it. You can custom order the cards to say whatever you want. Mine say, "Hello, I am a passenger on your flight today. I have a severe allergy to nuts, and if I breathe in nuts or nut products I will have a reaction requiring medical attention. I would like to ask that you please refrain from eating products containing nuts during this flight. I am very thankful for your cooperation. Have a nice flight!"
I pass these out at the gate area, about 45 minutes before the flight. I walk around asking people if they are going to my destination, for example, "Are you going to Boston today?", and if they say yes, I say, "Thank you" and hand them a card.
Since airlines won't accomodate my allergy, I only fly the ones who don't serve peanuts, such as Jet Blue. They used to make an announcement, but now they won't, so I make my needs known through these cards. I am VERY polite to everyone when I pass the cards out, and so far everyone has been supportive. It's not a guarantee, but if 90% of people listen and comply, at least only 10% of the plane poses a threat. It helps me to know that everyone knows, and so far, the airline has not had a problem with me doing this.