This may seem like a really naive question, but is buying a "green ticket" on an aeroplane flight something that is actually worth doing? How exactly does it work? What would be the most efficient way of offsetting your carbon emissions?

I assume not. Relevant: http://robertwiblin.com/2012/03/25/why-local-food-is-not-effective-altruism/

I'd suspect that almost any time you could pay money to offset something you're doing, you could be more effective by giving that same money to a top charity.