I think number one is coming to peace with the fact that you're not going to change people, and the best you can do is change how you interact with or mentally respond to the annoying people in your life.
With the example of the airline seats, I think you need to stick to your guns and tell the person you don't want to move Sometimes replying with a "no thanks", or "that's ok, I'd rather stay here" is enough to get the uninitiated to back down. If they then persist to get into a debate with you about why, you again have to stand strong and tell them it's your seat and you're not going to discuss it. Then they have to deal with it.
I think one of the thoughts that has helped me with annoying people is realizing that they are with themselves every second of the day, interacting with everyone else in their lives. If this person is annoying you, getting you riled up, or generally pissing you off, they are likely inducing the same feelings in everyone else they meet. Just imagine what their life is like, and rest assured that they will likely get their comeuppance