In looking for resources, I came up dry. There just isn’t that much out there on learning to encourage your spouse.
That said, there have been plenty of times I’ve asked God to teach me because I couldn’t find immediate answers.
We can still become experts at encouraging our spouses.
Mostly I think it has to do with becoming a student of your spouse and doing a lot of practice. The more you know your spouse, the easier it will be to say those things that bless and lift them up.
Keep up the good work!