For the longest time, I refused to vote on the grounds that the system was broken and corrupt and, while I was implicitly supporting the broken system by living in this nation without attempting to stage a revolution, I didn't want to explicitly support the broken system by actively participating in it. Although I retain my convictions that a) the system is broken and b) the major parties are both corrupt, I've become much more pragmatic over the years. I take it as a given that my government is going to take many of the principles I care most about, drag them through the mud, step on them, and leave them out in the rain. Having accepted that difficult truth, it's mush easier to accept the notion that I can participate in a corrupt political system in order to nudge my government toward behaving ever so slightly less monstrously.
It boils down to this. I'm very much a civil libertarian, an internationalist, and an economic progressive. Neither major party reflects my views, and third parties are, in a winner take all system, largely a waste of time. However, I'm willing to vote for the party least likely to take an authoritarian, nationalist, economically exploitive view of the way a country ought to be run, and that's the Democrats. They may take my ideals, place them face down, and violate them without benefit of lubrication, but the Rebuplicans do the same, and wipe their dicks on the curtains afterwards.
The idea is to limit the damage done by the corporate parties while we find a better way to choose our leaders. Voting is one part of it, to keep the party likely to do the least amount of damage in power (whichever party that may be, according to your particular views, but the correct answer is the Democrats
)...more importantly, I try to be politely outspoken about my ideas, to get them into the public consciousness, and to encourage others to do the same. Hoefully, in the not too far off future, an alternative party will be able to gain enough support to displace one of the big teo, and then, maybe, just maybe, that party will be able to enact true election reform, so that we aren't stuck with two bland corporate conglomerates to vote for.
That was sort of rambling, and probably doesn't make much sense...