Recently I've offset my carbon from a cross-country flight and a train trip from Los Angeles to San Diego. It is really easy to do. Airlines such as Virgin America and national train operator Amtrak provide a link to Carbon Fund after you purchase your tickets.
And you even get a brand new e-certificate to show off to all your friends on Facebook that you donated to help fight global warming by offsetting your carbon footprint.
Click here for the link to Carbon Fund to find out more information.