If you live in a "safe" state (CA, WA, ...), where one of the two big candidates is effectively certain to win, you can vote for your first choice third-party candidate in a way which might help in the long run -- once e.g. the Libertarian Party gets 5% of the vote in an election, it gets federal matching funds, and will be a lot harder to exclude from the process.

In the long run, I think I'd take the worst of the D/R candidates over a string of 8 consecutive elections if the consequence is ending the two party stranglehold.

The way the system works, without proportional representation, you probably will ultimately end up with two parties, but I'd rather replace both current parties with new competent-but-ideologically-distinct parties (like, say, a Green/Socialist/Union/Interventionist/etc. party vs. a Libertarian/Free-Market/Business/Isolationist party).

There are a lot of us that want a third party, but IMO it's just not going to happen without changing the voting system.

If you don't vote for them, it's guaranteed they won't get traction.

If you're reading this and live in California, you should strongly consider the 3rd parties on the ballot. Obama's got the state locked up by 15-20 points, so there's no risk in third party voting. Obama will win, Romney will lose. This will happen however you vote.

So take a look at the 3rd parties, and if one of them more closely matches your views, support them. Don't waste your vote on parties you don't believe in.

Right, but especially in the legislature, a cohesive group can pretty much take over one of the two big parties (1994 Gingrich, 2010 Tea Party).

Parties also can implode and die (Whigs, Federalists, ...)

I consider voting for a third party to be more influential. When any third party gets any significant tracion, the two major parties start revising their platform to bring in those third party voters. Instead of pushing forward the status quo and giving the main parties more confidence in their stance, you're causing them to think more about what they might change.