How can wives encourage their husbands?

The first thing I would say is pray earnestly for your husband. We are so prone to instruct, and it's called nagging when it gets to a certain point.

The second thing I would say is affirm every good thing that you see. Be verbally affirming of every work of grace in a husband's life.

And thirdly, from time to time in moments of sweetness, say, "Could we talk about the way it's going?" In other words, if she has concerns and he is not doing what he ought to be doing, instead of striking out at the moment the failure happens, find a neutral time when things are good between you and say, "Could we just talk a little bit about this?" Then bring up the issue at a moment where he might have the emotional resources to acknowledge the difficulty, instead of when he has just failed and is likely to bristle with self-defense.

So pray, affirm the evidences of grace you see in his life, and find the most helpful and hope-filled moments to talk about the short-comings.