Any other means of transportation is likely to be better. Can you possibly take a train or carpool?
Book non-stop flights, they are much more fuel efficient: less miles, and less climbing.
Every extra pound on a flight means more fuel to carry you -- pack lightly. The more people on a flight, the less fuel per person -- so flying coach counts too. Pee Green »Someone somewhere realized that if you pee before you board, you'll be just a bit lighter!
Take public transit rather than a vehicle, or tell the event about Spaceshare's ride-sharing services. Conference colors »Carrying materials related to your conference just might lead to a last-minute connection to share a taxi, shuttle, or make public transit social.
Ever more popular as an answer to the pollution of flying, carbon offsetting is coming to resemble Indulgences. more »Carbon offsets are a token: a great way to measure how much impact you've had, but they don't really undo it. If you want to make a donation to an environmental cause, they're often one of the least efficient uses of your money -- support a local cause you know something about; buy more organics!
More details about Offsets are above.
We looked. We have yet to find an airline that is making consistent, significant greening efforts