The lesson I took away from 2000 was that your vote almost never matters in any measurable way in determining the outcome of a national election.
All I have to do is look at my own situation in the last election.
I mean, look. That election was amazingly close. The difference in the popular vote for Gore and Bush was almost (or maybe even actually) statistically insignificant. Not only was the election close on a national scale, but in some states it was even closer, so that in Florida the question of how to deal with counting the votes had to go all the way up to the Supreme Court to be resolved. Clearly, if my vote was going to count in any national election, it was going to be that one.
But did it? No. I voted in a state that went for Gore over Bush by a ratio of about four to one. Even if I and everyone I know had stayed home, it wouldn't have made the slightest bit of difference to the outcome.
Sure, some peoples' votes mattered. If you were on the Supreme Court, your vote mattered. If you were in Florida and weren't turned away from the polls because you were incorrectly identified as a convicted felon, then your vote arguably mattered. If you were in one of the other swing states, sure, your vote arguably mattered.
But that's rare. Most stats aren't swing states, and in many elections most swing states don't matter. So most of the time, for most people, their vote doesn't make a bit of difference.
With all that said, I still vote, every chance I get. Why do I bother?
Well, I'll admit out front that it's not wholly rational. Some of it is upbringing; it's a civic duty! How can you not vote? That would be wrong! And I would also feel dumb if I didn't vote and the other guy won, even though rationally I could look back and see that my vote wouldn't have changed anything.
There is one reason to vote, though, which I wish more people understood, which is that is that local politicians tend to pay more attention to areas that have high voter turnout. If nobody votes in your district, the government is probably not going to do as much for you as they would if you were in a district where many people vote. This is true whether the people and issues your district goes for actually prevail in the election or not; if you can get the votes out, politicians will be more inclined to be nice to you, or at least not completely piss you off.
A more irrational reason, but one which I feel like could be turned into a more rational argument, is the feeling that if absurdly few people voted then it would be another thing that tended to result in the country going to hell. If only three people in the entire country voted, the country would be subject entirely to their whims. And, sure, the candidates that those three people voted for would be largely similar (because they would be trying to appeal to the same three people), and the other two hundred million people in the country could use that as an excuse to not vote, but something really unhealthy would be going on there. So all else being equal I think that the country as a whole (and individual members of the country) are better off when more people vote, so go ahead and vote. This is a lot more abstract than the last reason, though, and there are probably a lot more holes that can be poked in it.
Another reason is completely irrational, but I'll say it anyway because it's one that I tend to think about when I think about the political system in America. And that is that the political system in America is really designed to discourage high voter turnout, and that the political system benefits from this. (If you wanted to encourage voter turnout you would reward states or districts that had a lot of people turn out, but Rhode Island gets the same number of electoral votes whether there's 100% turnout or there's 6% turnout [as there was yesterday].) Politicians by and large benefit from this because if there's a lot of voter apathy then they can pretty much do whatever they want. Campaigns more and more focus on getting people to not vote for the other guy -- attack ads are known to reduce voter turnout, but they remain a very popular and potent political weapon. Politicians may decry low voter turnout, but that's what the whole system is based on.
So screw 'em. Go out and vote.