My personal take: (every "you" is the generic you, for whatever it's worth)
Every election presents you with a choice, and it's usually not the choice you'd like, but it is the choice you have to work with.
No matter how small the difference, if there is a difference you can detect you should always choose the better option even "better" means "slightly less bad".
But the choice you have on election day doesn't come out of nowhere.
Consider Obama. He's going to be on the ballot in 2012. That journey started in 1996. Now that's a pretty fast rise, I think, but even so it shows you a bit of how long it's going to take to get better options at the top if you start the work today.
Presidential candidates don't come out of nowhere, they come from the ranks of senators and governors and congresspeople, and members of state government. (The last President who didn't hold a previous elected office was Dwight D. Eisenhower.) And most of the offices they came out of were not ones people generally get into in their first dabbling in politics.
Not voting for the lesser evil won't get better presidential candidates, it will just increase the chances of the greater evil getting in.
To get better presidential candidates, also better senatorial candidates and better house members, what needs to happen is to start electing better people to the lower levels. Whether better Democrats, or independents/third party members who are better than Democrats. Because that's how change is actually going to happen.
And if you have the time, inclination, and energy, you can work on that right now. If a movement did that then it could completely rewrite American politics. (Actually, a movement is trying to do that right now, but given that they're a sort of tea-party/Ron Paul coalition, I don't think it'll change things for the better.)
But it's going to take time.
In the interm we still bear responsibility for our choices. That includes doing as little harm as possible. There are differences between the two parties, and those differences are a matter of life and death for some people, and though I might never meet the people who would be killed by my decisions, it is still my responsibility to make sure those decisions inflict as little suffering and death upon the world as possible.
But, as I said, that's just how I see it.