Probably this has already been said somewhere. If someone points out a good reference to me, I'll put in a pointer --
Under our primitive voting system, where the votes of losing candidates aren't rolled over to the voter's next choice, third-party candidates create a well-known problem: They may offer platforms better than those of the dominant parties, but the majority of the electorate will vote for the latter, and insofar as some don't, the third-party candidates' effect is to draw votes away from the candidates with a position most like theirs.
As long as they stick to the unrealistic position, "I am going to win this election", they cannot admit that this problem exists, and so cannot offer a solution. But if they can turn off their hubris, and admit that their likelihood of winning this particular election is low, I think that they can do something constructive. Run their campaigns, stressing the issues they think important, but making it clear that as the election approaches, if the fraction of voters who say "This is the candidate I would most like to see win" is not sufficient to elect them, then they will urge their supporters to vote for the best of the candidates who have a chance; perhaps giving their advice on who they recommend.
If a third party follows such a policy election after election, they can hope to see their support growing, and hope that there will come a turning point, when they see that they are likely to win, and that the time has come not to call it quits.
A corollary of these ideas is that such candidates should publicly urge opinion pollsters to ask, not only "Whom do you expect to vote for?", but also "Which of the candidates would you most like to see win?" It is that number which they can hope to see growing in the earlier years if they do a good job. When the polls show that number approaching a plurality, then the voters can see voting for such a candidate as a valid option.
(Of course, a system of preferential voting would do the whole thing more nicely; but a third-party candidate generally does not have the leverage to get that put into effect.)