If you're going to vote for the LibDems, is it because you actually want them to govern, or merely want them to govern more than you want the Tories or the other alternatives? If you you're going to vote for AV, is it because you'd really like to be ruled by people who will be elected under that system, or because you hate that idea slightly less than being ruled by the people likely to be elected under FPTP? Are you, in any sense, being presented with an option that you positively want and endorse?
Every time you turn up and tick one of their boxes, you are legitimating the government. You are preserving the illusion that the goverment governs by the consent of the governed because it has a popular mandate expressed through the ballot box.
It is not a waste of your vote to withhold that legitimacy, if it is not something you wish to confer. It is not a waste of your vote to withhold that consent, if all you can think to do with it is submit to the lesser of several evils. The BNP will not really end up running the country, whatever you do or do not do. And if everyone did it; if no-one turned up to the polls; if the mandate and the legitimacy were withdrawn; then there would the opportunity for actual, meaningful political change.
I leave you, as always, with the definitive words on the subject by Douglas Adams from So Long and Thanks for All the Fish.
"It comes from a very ancient democracy, you see..."
"You mean, it comes from a world of lizards?"
"No," said Ford, who by this time was a little more rational and coherent than he had been, having finally had the coffee forced down him, "nothing so simple. Nothing anything like so straightforward. On its world, the people are people. The leaders are lizards. The people hate the lizards and the lizards rule the people."
"Odd," said Arthur, "I thought you said it was a democracy."
"I did," said Ford. "It is."
"So," said Arthur, hoping he wasn't sounding ridiculously obtuse, "why don't the people get rid of the lizards?"
"It honestly doesn't occur to them," said Ford. "They've all got the vote, so they all pretty much assume that the government they've voted in more or less approximates to the government they want."
"You mean they actually vote for the lizards?"
"Oh yes," said Ford with a shrug, "of course."
"But," said Arthur, going for the big one again, "why?"
"Because if they didn't vote for a lizard," said Ford, "the wrong lizard might get in. Got any gin?"