I am becoming increasingly conscious of this. This is the hardest individual action for me. I love travelling, and I want to do more of it. If someone said that I could either never get on a plane again or never eat meat again, the decision would be so easy for me. I'd never eat meat again.
One simple thing I'd happily promise to never do again is to fly internally within the UK. Admittedly, I've only done this twice (Birmingham to Edinburgh and back, and Edinburgh to Glasgow and back) but I go to Scotland often for work and most of the time I drive or get the train depending on how much equipment I have to take with me. In all honesty, it probably doesn't take that much longer for me to get to Glasgow from mine via train than getting from my house to Birmingham Airport and flying to Glasgow, yet the environmental impact of flying is substantially greater.