We do it daily. This past year we learned about a great communication technique called mirroring. There is a very specific dialogue to follow, and it's helped us talk calmly about things we used to argue about.
Anyway, part of our mirroring training (since it's so weird and different, and can be hard to remember about in the heat of a discussion) is an Appreciation Dialogue we do every night before bed. We tell the other 1 thing we appreciated about them that day. It helps us get more comfortable with the mirroring technique (so we can use it more effectively during disagreements) and it's a GREAT way to express our love and gratitude for each other.
It has been a WONDERFUL addition to our nightly routine and to our relationship.