Nowadays it is possible to calculate how much carbon dioxide has resulted from your air travel and other activities.
It's misleading to think you can ever truly offset this, but at least by supporting projects such as tree planting or renewable energy programmes, you can help do something to reduce carbon emissions.
A number of other sites enable you to calculate the environmental costs of your travel, including the National Energy Council and the Travel Calculator.
Climate Care calculates how much carbon dioxide your journey has created and then invites you to donate a corresponding amount to their carbon dioxide reduction campaigns - e.g. clean, hydroelectric power for Bulgaria, low energy lightbulbs for Mauritius and restoring rainforest in Uganda.
The Carbon Neutral Company helps organisations measure and reduce carbon dioxide emissions. They support tree planting, energy efficiency and renewable energy projects around the world.
But please remember in all this, it's far better not to make the carbon footprint in the first place!