"Politicians are like nappies. They should be changed regularly, and for the same reason." - Ken Dodd
It is beyond argument that democracy is the best form of government that humanity has implemented so far. It may not be the best possible form of government (see here, for example), but it's the best we've tried. Democracies are the richest societies on Earth; their populations have the longest life expectancy; and they have the worst immigration problems (which is another way of saying that everyone else wants to live in them).
But there is a superficial paradox: democracies are intrinsically inefficient compared to, say, dictatorships in the same way that a mob is less efficient than a disciplined army, and for the same reasons. So why should an inefficient form of government work best? The obvious answer - that government in itself is a bad thing and that less of it is therefore better - falls at the first counterexample: Somalia has no government at all, and it is one of the most unpleasant places to live in the World.
Democratic politicians are no less venal and corrupt than politicians working in other forms of government (read any newspaper for proof); indeed it is reasonable to suppose that much the same people would be running the government regardless of the political system under which they found themselves operating. Think of any minister in your government and visualize him or her serving under Robert Mugabe ("I think it's best to work for change from within."). It's not a big leap of imagination, is it?
No. The reason that democracy works is not because it puts the right people in government. There are no right people to be in government because no human being - you, me, Barack Obama, Wen Jiabao; none of the seven billion of us - has the faintest idea how to run a country. (We merely all have opinions about how it should be done, which is not the same thing at all.) The reason that democracy works is because it has a solid mechanism for removing people from power.
Having no government is bad (Somalia). Government by the same people for a long time is bad (Zimbabwe). But high turnover among governors is good.
It follows that, in an election, we should all ignore the record of the incumbents, we should all ignore the policies of the candidates, and we should all ignore their personalities.
We should simply vote in the way that is most likely to remove the current lot (whoever they are) from office.