If you must choose between a popular candidate whose ideals do not align well with yours, and a candidate who is very unlikely to win the election, but whose values you like, which do you vote for? Do you think that voting for a candidate who cannot win is "wasting" your vote?
This is a very difficult issue, because if everybody thinks that voting for a fringe candidate who is nonetheless somebody who has similar values and ideals to you is a "wasted vote," then this calls the whole system of democracy into question. Perhaps we need to reassess our idea of what a "wasted vote" is. Certainly we must be pragmatic, but at the same time, democracy is about us having the opportunity to say what we want, and voting allows us to do this. Even if your candidate does not win, you have still had that opportunity.
In theory, if enough people are willing to vote for a fringe candidate, then more mainstream candidates have to consider adopting some parts of their platform. Tea Party candidates didn't fare that well in the 2010 midterms, but they did force their ideas, whatever you make of them, on the agenda for the mainstream Republican Party. In any case, I have always voted my conscience.
I don not think that one ever wastes a vote. by voting, one is speaking their mind about a candidate. Regardless of the winner, one's vote is important. By not voting, one is simply saying that they don't care what happens. Without a vote, one cannot say anything about what happens. That being said, not all candidates are likely to match perfectly with one's opinions. My suggestion would be to choose the one which most closely matches one's own ideology.
Much might depend on the particular electoral circumstances. If it was likely that the incumbent might be reelected with a large margin in any case, no matter how one voted, then the temptation would be to vote purely according to one's convictions. However, if the election was likely to be "close," then it might be wise to vote pragmatically.
Like post 3 states, there are too many variables to have a clear principle. Hence, the best approach might be to view the issue though a different framework. Perhaps what you can do is to have a cluster of issues that you believe are important. In light of this cluster, vote for the candidate that is closest to the cluster. However, it is important to have a pragmatic point of view, so that you do not "waste" your vote on someone who would never win.
The bottom line is that it's your vote, you can choose to use it in any way you see fit, or not to vote at all. One of the beauties of this democracy is that it allows you to choose how or whether you will participate, and to what degree. I believe a person should vote for whomever matches their ideals most closely. This makes the "mainstream" candidate more likely to include planks from the minority support candidate in their platform. Or, enough of a groundswell might come for that candidate that he/she becomes the mainstream. It's a win-win either way.
For me, trustworthiness is the personality trait that is most important in choosing a candidate (local, state or national) for whom I will give my vote. I am a Democrat, but I never voted for Bill Clinton because I didn't trust him. He certainly had issues in this regard, but in retrospect, I believe he is one of the finest presidents in the past 50-75 years. I found no reason to trust either of the Bushes (the first Bush, with his CIA past, was a no-brainer), and both of their presidencies were filled with deceitful acts against the American people. I have voted for many Presidential candidates who had little chance of winning--George McGovern and Walter Mondale come to mind--and I didn't consider them wasted votes. As for the upcoming Republican primaries, I don't believe any vote would be considered wasted if the candidate fulfills one's ideals for leadership. I would NEVER vote for such a scoundrel as Newt Gingrich, and I can't really imagine why anyone who knows anything about his past would do so. I particularly like Ron Paul, and as of today (December 19), he is leading in the Iowa straw poll, so what might have been considered a wasted vote several months ago would not be today. Votes for third-party candidates, such as George Wallace and Ross Perot, are not wasted votes, IMO; and if Donald Trump decides to run in 2012, the votes he receives will probably decide the election--for either Obama or the Republican candidate.
I think that it depends on what your options are. If you are voting in the GOP primary, for example, and you really like Santorum but think Romney is okay, then I'd say you vote Santorum. If that dilutes the anti-Romney vote and allows him to beat Gingrich, it's no big deal because Romney is acceptable to you.
But let's say you like Huntsman and hate Gingrich. You'd better vote for Romney in that case because you wouldn't want to dilute the anti-Gingrich vote and allow him to win.
So I don't think there's a cut-and-dried rule. I think it depends on what consequences your vote for your ideal candidate might have. If such a vote helps an unacceptable candidate to win, I'd think you wouldn't want to do it.
I do not think one is wasting ones vote if one votes from conviction and principle, even if his/her candidate has no chance of winning. There are those who urge us to consider the practical consequences of our vote; yet to me to vote for a candidate with whom one does not agree but simply because he/she can win is somewhere between prostitution and horse track betting. I not only have to live with my own conscience, the day will come when I may need to defend my vote, to my son or grandchildren.
I recall Rep. Henry Hyde during the Clinton Impeachment remarking:
In the last analysis, our integrity is the only thing we carry to the grave."
When that day comes, I want to leave here with a clear conscience; in political matters as well as others.