That's the attitude that perpetuates the issue with other parties being perceived as not viable. If you decide not to vote for them because it's throwing your vote away, then you just continue the cycle. It's not about voting for who you think is the lesser of two evils, but about who you feel is the best qualified to do the job as you believe it should be done. Call me an idealist if you like, but that's how I see the issue. You're only throwing your vote away if you view it like that. If you don't, then you're sending a statement that you don't like the candidates that were presented by the two major parties, and if enough people actually stand up and vote their convictions, then perhaps it can affect a change.
It's difficult though - I do understand that of course if we all voted for the parties that made the most sense rather than the big runners, maybe things would change. But when you know that voting for a smaller party in an upcoming election will amount to absolutely nothing, it's hard to think it's worth it.
I still vote, and generally I vote for one of the main contenders because I've always found one that agrees with my ideals. But I understand why some people might think if they don't agree with any of them, what's the point? The majority will vote for them, and that's not likely to change anytime soon.