As the opportunity to vote for something rolls around once again, I only ask that you consider the following questions.
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 And if everyone did it; if no-one turned up to the polls; if the mandate and the legitimacy were withdrawn; then there would the opportunity for actual, meaningful political change.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.