At least for flights around Europe, taking the train. Flight in Europe are very cheap. It's actually cheaper to fly from London to Newcastle than it is to get the train, and it takes roughly the same time.

For all flights the alternative is just not to go. Instead of flying to the Caribbean for a holiday, go some place closer and take the train, instead of flying for a business meeting, just call in. If it's worth that much for you to fly, then paying a little tax on top should be fine. Flyers still aren't paying fully for the damage they do to the environment.