Judging by this evening's Des Moines Register numbers, I suggest that people vote for Cruz in Iowa unless new information comes along to change the picture significantly. Rubio should be the nominee, for reasons I've explained elsewhere, but giving Trump a loss in Iowa is more important than giving Rubio a better third-place finish. And I don't know how voting for Carson, Paul, Christie, etc. could be justified by their supporters. They ought to support Cruz as well.
But if you read the article I linked in my first sentence above, you'll see that Trump has a lot of problems. Half of his supporters haven't participated in the caucuses before. And there's a consensus that Trump doesn't have much of an operation in place for getting people out to vote, whereas Cruz seems to have the best operation. I still expect Cruz to win Iowa, but a Trump victory is too much of a threat for any of you in Iowa to vote for anybody other than Cruz.
By the way, Gallup recently published an article about how Trump has the worst favorability numbers in Gallup's nearly quarter-century history of tracking favorability numbers for presidential candidates.