Today, Londoners will vote for the next Mayor. I won't be among them. I don't think that voting is any more one's "duty" than supporting a football team. Indeed, in large-scale elections (like the Mayoral election) it is probably a lot less important than cheering on your favourite team. If your aim is to affect policy, voting is irrational. If you want to act ethically, voting is irrelevant.
Mathematically, the chances of a single vote actually determining the outcome of an election in a meaningful way (that will affect policy outcomes) is infinitesimally small (ten million to one for voters in US swing states in 2008). The closest general election result ever in the UK was in 1997, when Lib Dem Mark Oaten was elected by two votes. Not only would one vote still not have made a difference here, the result was annulled and a new election held in which Oaten won by a landslide. Even if one vote did make a difference, the result would (apparently) be annulled.
This was with a turnout of 62,000 – the larger the turnout, the less likely it is that the vote will be tied or won by a single vote. In the last London Mayoral election, turnout topped two million. The chances of a single vote determining this election are extremely slim – less slim than they'll be at the next general election, to be sure, but still very slim. (Incidentally, some people will always reply to this, "If everyone thought that way, nobody would vote!". This is silly. We act marginally as individuals. If everyone went to the cinema tonight, there would be no room for me. This is a bad reason not to go to the cinema.)
Voting isn't instrumental, aimed at affecting policy, it's expressive. Like supporting a football team from home, you do it because it makes you feel good, not because you think it'll make the team win. But there isn't really much reason to care who wins the Mayoral election. The Mayor's powers are extremely limited, and the differences between Boris and Ken (the only two people who have a hope of winning) are more about personality and style than policy. Sure, Ken gives me the creeps, but Boris has disappointed me by resorting quickly to spending pledges whenever he's challenged about things like transport or policing. Maybe that's what it takes to win, but don't expect me to care enough about you to go out and vote for you if it is.
Some people like to say that voting is everyone's duty as citizens in a democratic country. This is nonsense. States establish and enrich themselves by violence. Don't believe me? See what happens when you choose not to pay all your taxes because, say, you don't want to use the NHS. You have no more of a duty to vote than you have a duty to go down to your local Mafia's consultation meeting about how to make protection money collection as pain-free as possible.
Voting is a remarkably poor tool in determining socially-optimal outcomes. When I go to the bookstore, I can pick from thousands of different books and even pay more to buy obscurer titles. Even if I'm in a minority wanting to learn Polynesian nose flute-playing, I can pay extra to express the intense wish I have to learn, which will allow my minority wishes to go fulfilled.
Voting can't measure intensity of feeling like markets can. It's blunt, uniform and heavily weighted towards the wishes of a majority that often chooses poorly. Democracy is a bookstore where you can have any novel you want, as long as it's Twilight.
To be fair, things might be different in your local elections, where one vote sometimes can make a difference. That's a good argument for more localism, so democracy can be a little less tyrannical and local government a little more attractive to talented people. But in this large Mayoral election where the candidates are offering, more or less, the same thing, there's no point. Instead of trudging down to the polling booth today, I'll be going home and reading a good book. I suggest you do too.