Short answer
Affection can be maintained through appreciation, kindness, shared experiences, communication, small gestures and continued interest in each other's lives.
Islamic Guidance
What Islam teaches
Allah describes affection and mercy between spouses. Good treatment remains important throughout married life, not only during the early years.
Practical Guidance
What you can consider
Continue saying thank you, spend time together, celebrate important occasions appropriately and avoid treating the spouse's contributions as automatic or unimportant.
Sources
Qur'an 30:21; Qur'an 4:19.